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CONTRIBUTION S 


TO 

MOLECULAR PHYSICS 

£ ' 

IN THE DOMAIN OF 

RADIANT HEAT. 


A SERIES OF MEMOIRS PUBLISHED IN THE ‘ PHILOSOPHICAL TRANSACTIONS 
AND «PHILOSOPHICAL MAGAZINEWITH ADDITIONS. 


, BY 

JOHN TYNDALL, LL. D., F. R. S., 

1 $ 

PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION. 


> f 

> ) ) 

> , > 

NEW YORK: 

D. APPLETON AND COMPANY, 

549 & 551 BROADWAY. 

1873. 


QC33I 

Tas 

l«73 


By Transfer 
JtIN i i &f 






HENRY BENCE JONES, M.D. D.C.L. F.R.S. 


HON. SEC. R. I. 


If unswerving devotion to tlie Royal Institution, firstly, 
and above all, as a school of original enquiry, and 
secondly as an organ for the diffusion of scientific 
knowledge, merit the grateful recognition of its Members 
and its Professors, then justice ought to require no 
stimulus from friendship, in associating these Researches 
with your name. 

They were one and all conducted on the spot 
whence, during sixty years, issued in unbroken succession 
the labours of Young, Davy, and Faraday. Would that 
they were more worthy of their immortal antecedents ! 

t 

JOHN TYNDALL. 


Royax Institution : 
May 1872. 







PBEFACE. 


In the Preface to the Third Edition of my work on Heat, 
written in January 1868, the hope was expressed that before 
the end of that year the original Memoirs which I had 
contributed to the 4 Philosophical Transactions,’ and other 
journals, during the previous eighteen years, would be pre¬ 
sented to the scientific public. Hitherto this hope has 
been only partially fulfilled by the publication of the 
researches on Diamagnetism and Magne-crystallic Action. 

The present volume contains the Memoirs on Eadiant 
Heat, considered as an explorer of Molecular Condition. I 
have read them over carefully, and have tried to augment 
their clearness without altering their substance. 

* 

In front of each memoir is placed an analysis of its 
contents, from which the reader can at once learn the 
nature of the inquiry. I have also added here and there 
some necessary historic data. 

The points of difference between the late Professor 
Magnus and myself regarding the action of air and that 
of aqueous vapour on radiant heat are placed in their 
proper sequence and relation. At the end of the series of 
Memoirs the discussion is resumed, and brought, I trust, to 
a fair conclusion. 




Yin 


PREFACE. 


I ought to inform the reader who desires but a partial or 
general acquaintance with these researches, that summaries 
of most of them have been already published in the 
various editions of my work on Heat. 

Finally, I would offer my b*est thanks to the Council of 
the Royal Society for the ready courtesy with which they 
granted me the use of the Plates employed to illustrate 
these Memoirs in the ‘ Philosophical Transactions.’ 

John Tyndall. 


Royal Institution: 
Matj 1872 . 


CONTENTS. 


Analysis of Memoir I . 

I. On the Absorption and Radiation of Heat by Gases and 
Vapours, and on the Physical Connexion of Radiation, 
Absorption, and Conduction. 

Introduction. 

Sect. 1. The Galvanometer and its Defects.—Magnetic Analysis of its Wire 

2. First Experiments on Absorption by Ordinary Methods 

3. Method of Compensation. 

4. Final Form of Apparatus ........ 

5. Absorption of Eadiant Heat. First Kesults.—Action of Ozone 

and of Compound Gases on Radiant Heat ..... 

6. Variations of Density—Relation of Absorption to Quantity of 

Matter. 

7. Action of Sulphuric-ether Vapour on Radiant Heat 

8. Extension of Inquiry to other Vapours. 

„ 9. Action of Chlorine.—Possible Influence of Vapours on the Interior 

Surface of the Experimental Tube. 

10. Action of Permanent Gases on Radiant Heat . . . . 

11. Action of Aqueous Vapour.—Possible Effect of an Atmospheric 

Envelope on the Temperature of a Planet. 

12. Radiation of Heat by Gases. Reciprocal Experiments on 

Radiation and Absorption........ 

13. The Varnishing of Polished Metal Surfaces by Gases . 

14. First Observation of the Radiation of a Vapour heated dynami¬ 

cally . 

15. On the Physical Connexion of Radiation, Absorption, and Con¬ 

duction ........... 

Supplementary Remarks, 1872. . . . . . . • 

1. Note on the Construction of the Thermo-electric Pile . 

2. Note on the Construction of the Galvanometer . . . . 

3. Remarks on the different Values of Galvanometric Degrees . 

4. Calibration of the Galvanometer. 

Historic Remarks on Memoir I. ...... • 


PAGE 

2 


7 

7 

8 

11 

13 

15 

17 

21 

24 

27 

34 

36 

38 

41 

44 

44 

4G 

51 

51 

53 

55 

56 

59 







X 


CONTEXTS. 


Analysis of Memoir II . 

II. Further Researches on the Absorption and .Radiation 
of Heat by Gaseous Matter. 

Sect. 1. Recapitulation. 

2. New Apparatus. 

3. Preliminary Efforts and Precautions.—Chlorine, Ozone, and Aqueous 

Vapour.. • • ' • 

4. First Experiments on the Human Breath.—Chlorine and Hydro¬ 

chloric Acid.—Bromine and Hydrobromic Acid .... 

5. New Experiments on Gases ........ 

6. Radiation through Black Glass and Lampblack . . . . 

7. Selective Absorption by Lampblack. 

8. New Experiments on Vapours.—Further Proof of the Influence of 

Chemical Combination on the Absorption of Radiant Heat 

9. Superior Action at one Pressure does not prove Superiority at all 

Pressures ........... 

10. Dynamic Radiation and Absorption . 

11. To determine the Radiation and Absorption of Gases and Vapours 

without any Source of Heat external to the Gaseous Body itself. 

I.—Vapours .......... 

12 . Attempted Estimate of Quantity of Radiant Vapour 

13. II.—Gases. 

14. Influence of Length and Density of Radiating Column 

15. Laplace’s Correction for the Velocity of Sound.—Remarks on the 

Radiant Power of Molecules and Atoms ..... 

16. Action of Odours upon Radiant Heat. 

17. Action of Ozone upon Radiant Heat. 

18. Experiments of De la Rive and Meidinger. 

19. On the Constitution of Ozone ....... 

20. Action of Aqueous Vapour upon Radiant Heat.—Experiments of 

Professor Magnus .. 

21. Night-Moisture on the Interior Surface of Experimental Tube. 

—Abandonment of Rock-salt Plates ...... 

22. Proposed Solution of Discrepancies. 

23. Action of Atmospheric Envelope.—Possible Experimental Deter¬ 

mination of the Temperature of Space ..... 

24. Remarks on the Experimental Evidence of Gaseous Conduction. 

—Influence of Density on Convection.—Internal Frictiun of Air 


Analysis of Memoir III. 

III. On the Relation of Radiant Heat to Aqueous Vapour . 

1. Objections to Rock-salt Plates considered.—New Experimental 

Arrangement . . . . • • • 

2. Objection to Employment of London Air considered.—Radiation 

through Air from Various Localities ...... 

3. Radiation through Open Tubes ....... 

4. Radiation through Closed Tubes.—The Quantity of Heat absorbed 

proportional to the Quantity of Humid Air .... 

5. Radiation through the Open Air ....... 


PAGE 

66 

69 

69 

71 

72 

75 

80 

83 

84 

85 

88 

89. 


91 

93 

95 

96 

97 
99 

102 

103 

104 

105 

110 

114 

117 

118 


124 

127 


127 

128 
131 

133 

135 








CONTENTS. 


XI 


Sect. 6. Application of Results to Meteorology.—Tropical Rains.—Cumnli. 

—Condensation by Mountains.—Temperatures at Great Eleva¬ 
tions.—Thermometric Range in Australia, Tibet, and Sahara.— 
Leslie’s Observations.—Melloni on Serein . . . . 


Analysis op Memoir IY . 

IY. On the Passage op Radiant Heat through Dry and Humid 
Air. 

Analysis of Memoir Y. 

Y. On the Absorption and Radiation op Heat by Gaseous and 
Liquid Matter. 

Introduction. 

1. Further Experiments on the Power of Gaseous Matter over Radiant 

Heat.—New Apparatus.—Absorption by Gaseous Strata of 
different Thicknesses. 

2. Effect of an Atmospheric Shell of Gas or Vapour two inches thick 

upon the Temperature of a Planet. 

3. New Method of Experiment and its results—Division of Experi¬ 

mental Tube into two chambers.—Transmission of Radiant Heat 
through Gases in one or both. 

4. Influence of ‘ Sifting’ by Gaseous Media. 

5. Application of Method to Vapours. 

6. New Experiments on Dynamic Radiation.—Radiation of Dynami¬ 

cally heated Gas through the same Gas, or through other Gases 

7. Influence of Tarnish, or of a Lining on the Interior Surface of 

. Experimental Tube.—Dynamic Radiation from the Surface 

8. Radiation of dynamically heated Vapour through the same Vapour 
and through a Vacuum. —Influence of Length of Radiating Column. 

—Different Effects of Length on Gases and Vapours . 

9. First Comparison of the Actions of Liquids and their Vapours 

upon Radiant Heat. 


Analysis of Memoir YI. 

YI. Contributions to Molecular Physics. 

1. Preliminary Considerations.—Description of Apparatus 

2. Absorption of Radiant Heat of a certain Quality by eleven different 

Liquids at five different Thicknesses ...... 

3. Absorption of Radiant Heat of the same Quality by the Vapours 

of these Liquids at a common Pressure . 

4. Order of Absorption of Liquids at a common Thickness, and Va¬ 

pours at a common Pressure ....... 

5. Order of Absorption of Liquids and Vapours in proportional 

Quantities . .. 

6. Remarks on the Chemical Constitution of Bodies with reference to 

their Powers of Absorption . . . . . . 


PAGE 

137 

146 

149 

164 

165 

165 

166 
170 

172 

176 

179 

183 

186 

188 

191 

196. 

199 

199 

205 

209 

210 
211 


214 




Xll 


CONTENTS. 


Sect. 7. Transmission of Radiant Heat through Bodies opaque to Light. 

Remarks on the Physical Cause of Transparency and Opacity . 

8. Influence of the Temperature of the Source of Heat on the Trans¬ 

mission of Radiant Heat 

9. Changes of Diathermancy through Changes of Temperature.— 

Radiation from Lampblack at 100° C. compared with that from 
white-hot Platinum . . . . . . ... 

10. Changes of Diathermancy through Change of Source of Heat.— 

Radiation from Platinum and from Lampblack at the same 
Temperature 

11. Radiation from Flames through Vapours.—Further Changes of 

Diathermancy. 

12. Radiation of Hydrogen Flame through Dry and Humid Air.— 

Influence of Vibrating Period on the Absorption 

13. Radiation of Carbonic-oxide Flame through Dry and Humid Air, 

and through Carbonic Acid Gas.—Further illustration of Influ¬ 
ence of Vibrating Period ........ 

14. Comparative Radiation of Carbonic-oxide Flame through Carbonic 

Acid Gas and Olefiant Gas.. 

15. Radiation of Hydrogen Flame through Carbonic Acid Gas and 

Olefiant Gas .. •••••••• 

16. Radiation of Carbonic-oxide Flame through Carbonic Oxide and 

of Bisulphide-of-Carbon Flame through Sulphurous Acid . 

17. Radiation of the Flames of Carbonic Oxide and Hydrogen through 

Sulphuric and Formic Ether Vapours.—Reversal of Order of 
Absorption .......... 

18. Radiation of Hydrogen Flame, and of Platinum Spiral plunged in 

Hydrogen Flame, through Liquids.—Conversion of Long Periods 
into Short ones ......... 

19. Radiation of Small Gas Flame compared with that of Hydrogen 

Flame—Further Changes of Diathermic Position 

20. Explanation of Certain Results of Melloni and Knoblauch 

21. Radiation of Hydrogen Flame through Lampblack, Iodine, and 

Rock-salt.—Diathermancy of Rock-salt examined 

22. Physical Connexion between Radiation and Conduction 


Analysis of Memoir YII. 

VII. On Luminous and Obscure Radiation. 

1. Spectrum of Hydrogen Flame. 

2. Influence of Solid Particles. 

3. Persistence and Strengthening of Obscure Rays by Augmentation 

of Temperature ......... 

4. Persistence and Strengthening of Rays illustrated by means of 

a Ray-filter of Iodine and Bisulphide of Carbon . . . . 

5. Combustion by Invisible Rays ....... 

6. Melloni’s Method of determining the Ratio of Visible to Invisible 

Rays.—Diathermancy of Alum and of the Humours of the Eye . 


PAGE 

215 

219 

222 

224 

226 

229 

230 

232 

233 

234 

235 

236 

240 

241 

243 

245 

250 

253 

253 

256 

256 

258 

261 

263 


« 






CONTENTS. 


Xlll 


Analysis op Memoir YIII . 

VIII. On 'Calorescence, or the Transmutation op Heat Hays . 

Sect. 1. General Statement of the Nature of this Inquiry 

2. Source of Rays.—Employment of Rock-salt Train 

3. Methods of Experiment and Tabulated Results .... 

4. Graphic Representation of Results.—Curve of the Electric Spec¬ 

trum.—Deviations from Solar Spectrum ..... 

5. Rays from Obscure Sources of Heat contrasted with Obscure Rays 

from Luminous Sources of Heat.—Further Observations on the 
Construction of a Ray-filter. 

6. Invisible Foci of the Electric-light.—Efforts to intensify their Heat. 

—Danger of Bisulphide of Carbon, and trial of other substances. 
—Final Precautions ......... 

7. Calorific Effects at Invisible Focus.—Placing of the Eye there 

8. Improvement of Mirrors.—Exalted Effects of Combustion at Dark 

Focus ........... 

9. Transmutation of Heat Rays.—Calorescence . . . . 

10. Various Modes of obtaining with the Electric-light Invisible Foci 

for Combustion and Calorescence .f 

11. Invisible Foci of the Lime-light and the Sun . 

12. Relation of Colour to Combustion by Dark Rays . . . . 

13. Calorescence through Ray-filters of Glass.—Remarks on the Black- 

bulb Thermometer. 


Analysis of Memoir IX. 

IX. On the Influence of Colour and Mechanical Condition 
on Radiant Heat. 

1. Proof that White Bodies sometimes absorb Heat more copiously 

than Dark ones.—Explanation ....... 

2. Melloni on Colours, and Masson and Courtepee on Powders, in 

relation to Radiant Heat ........ 

3. New Experiments on Chemical Precipitates.—Influence of Colour 

and Chemical Constitution.—Sulphur Cement .... 

4. Tabulation of the Radiant Powers of Powders.—Employment of 

Electric Attraction instead of Sulphur Cement .... 

5. Qualitative Experiments.—Radiation of various Bodies through 

Rock-salt.—Unequal Diathermancy of the Substance 

6. Radiation of Powders.—Reciprocity of Radiation and Absorp¬ 

tion . 


Analysis of Memoir X. 

X. On the Action of Rays of High Refrangibility upon 
Gaseous Matter. 


1. Introduction .......... 

2. Theoretic Notions: Formation of Actinic Clouds through the 

Decomposition of Vapours by Light. 

3. Description of Apparatus. 


PAGE 

270 

273 

273 

275 

276 

279 

282 

286 

290 

293 

294 

296 

300 

302 

303 

308 

311 

311 

314 

315 

320 

321 
325 

330 

333 

333 

336 

338 







XIV 


CONTENTS. 


PAGE 


Sect. 4. The Floating Matter of the Air.• 

5. Deportment of Nitrite of Amyl 

6. Iodide of Allyl and Iodide of Isopropyl . . . • • 

7. Deportment of Liquids and of their Vapours towards Kays of High 

Kefrangibility. 

8. Influence of a Second Body on the Actinic Process 

9. Generation of Artificial Skies . . • ... 

10. Changes of Polarization in Actinic Clouds . . . . . 

11. Early Difficulties and Sources of Error.—Action of Infinitesimal 

Quantities of Vapour ........ 

12. Details of Experiments. 

13. Action of Kays of Low Kefrangibility. 


338 

342 

345 

346 
349 
352 
354 

357 

362 

374 


XI. Aqueous Vapour: Discussion Resumed. 

Analysis of Professor. Magnus’s Paper on Gaseous Conduction and 
Absorption. 

1. Gaseous Conductivity. 

2. Gaseous Diathermancy .... 

3. Proof of Convection. 

4. Experiments in Glass Tubes with Glass Ends 

Observations on Professor Magnus’s Paper ‘ On the Influence of 
the Absorption of Heat on the Formation of Dew ’ . . 383 


1. Explanatory Remarks.383 

2. Discussion of Paper.385 

First Remarks on the Paper of Professor Magnus .... 387 

Professor Wild’s Experiments.389 

Professor Magnus’s Last Paper.390 

Remarks on Professor Magnus’s Last Paper ...... 392 

Concluding Remarks and Summary.396 

Ice-making in the Tropics ......... 399 


. 378 

. 378 
. 380 

. 381 
. 382 


XII. Recent Researches on Radiant Heat.405 


XIII. On Radiation through the Earth’s Atmosphere . . .421 

XIV. On a Xew Series of Chemical Reactions produced by 

Light.425 

Action of the Electric-light.425 

Action of Sunlight.427 

Physical Considerations ......... 427 


Production of the Blue of the Sky by the Decomposition of Nitrite of Amyl 429 

XV. On the Blue Colour of the Sky, the Polarization of 
Sky-light, and on the Polarization of Light by Cloudy. 
Matter generally .. .431 


441 


XVI. On Cometary Theory. 

XVII. On the Formation and Phenomena of Clouds 


445 










LIST OF PLATES. 


*“♦ 0 * 


Plate illustrating Memoir I. on the Absorption and Eadiation 
of Heat by Gases and Vapours, and on the Physical Con¬ 
nexion of Eadiation, Absorption, and Conduction . Frontispiece. 


Plate illustrating Memoir XI. on Aqueous Vapour to face page 378 


































































' 



























































I. 


OX THE ABSORPTION AND RADIATION OF HEAT BY 
GASES AND VAPOURS, AND ON THE PHYSICAL 
CONNEXION OF RADIATION, ABSORPTION, AND 
CONDUCTION. 


1 


I 


ANALYSIS OF MEMOIR I. 


- - 4 0 *- 

The researches embodied in the following memoir were begun in the early 
part of 18o9; and the first notice of them is published in the ‘Proceedings of 
the Royal Society ’ for the 26th of May of that year. 

They arose in part from the desire to do for the gaseous form of matter what 
had been previously so well done by Melloni for the liquid and solid states of 
aggregation. I hev were also stimulated by the persuasion that not only the 
physical but the chemical, in other words the molecular , condition of bodies 
probably played a part previously unrecognised in the phenomena of radiation 
and absorption. 

At the time here referred to, the belief was general that as regards its rela¬ 
tion to radiant heat the gaseous form of matter was inaccessible to experiment. 
Of the published attempts in this direction, two only are known to me, the one 
by Melloni and the other by Franz. Both are referred to in the ‘ Introduction* 
to this memoir, and are there stated, I believe correctly, to have left this field 
of inquiry ‘ perfectly unbroken ground.’ 

The memoir naturally begins with a description of the instruments employed; 
the difficulty of obtaining a galvanometer-coil free from magnetic action is 
dwelt upon; and a simple method is proposed of testing the galvanometric 
purity of copper wire. It is shown that by an experiment of a moment’s 
duration we can satisfy ourselves, before the coil is constructed, whether the 
wire is fit for galvanometric purposes or not. 

Following the methods of observation introduced by Melloni, experiments on 
air and other gases were executed and recorded. They proved conclusively 
that to grapple with these gases far more powerful sources of heat than any 
previously employed would be necessary. Hence arose the problem how to 
employ such sources, and at the same time to maintain the needle of the galvano¬ 
meter in a condition as sensitive as if no heat at all were falling upon the 
thermo-electric pile. 

The problem is thus solved:—Two sources of heat are permitted to radiate 
against the two opposite faces of the pile. However powerful these opposing 
radiations may be, if they be only equal, they neutralise each other, and permit 
the needle of the galvanometer to point tranquilly to zero. Between one of 
these sources and the pile a wide tube is introduced, which can be exhausted 
or filled at pleasure with any gas. Supposing the opposing forces to be equal 
when the tube is empty, the introduction of any gas, capable of quenching even 




ANALYSIS OF MEMOIR I. 


3 


an infinitesimal fraction of the heat, would give the opposite source the mastery; 
a deflection of the galvanometer would follow, and from the magnitude of the 
deflection the amount of heat quenched by the gas could be immediately 
deduced. 

The final form of the apparatus thus sketched in outline was determined by 
a long course of tentative experiments. The possibilities of error were numerous, 
both in the arrangement of the apparatus and in the selection and purification 
of the substances to be examined. Experiments are recorded which show the 
infinitesimal action of the elementary gases and the perfectly enormous action 
of some of the compound gases upon radiant heat. To render these contrasting 
results secure ; to guard against instrumental defects which might readily sub¬ 
stitute a delusive for a real action ; and to avoid impurities which, though 
infinitesimal when measured chemically, were found competent in the case of 
the feebler gases to entirely vitiate the results, some thousands of experiments 
were executed. 

As regards instrumental arrangements, for example, it was proved that 
to avoid errors arising from convection, and from dynamic heating and chilling, 
it was absolutely necessary that the gases examined should not come into 
contact either with the radiating source or with the face of the thermo¬ 
electric pile. It was necessary that the heat lost by the one and received by 
the other should be purely radiant heat —a result which could never be calculated 
upon if contact were permitted with either the pile or the source. 

One of the earliest trials with the new apparatus, in which electrolytic oxygen 
was employed, proved the modicum of ozone which accompanied the oxygen to 
exercise a far more potent action upon radiant heat than the oxygen itself. In 
§ 19 Memoir II. this result is further developed, the constitution of ozone now 
generally agreed upon being deduced from these developments. 

While the action of oxygen, hydrogen, nitrogen, and air, even at the full 
pressure of the atmosphere and subjected to the most powerful tests, proved 
barely measurable, amounting certainly to not more than a fraction of a unit 
per cent, of the incident heat; experiments on olefiant gas are recorded in which 
the quantities employed varied from j^L_th of an atmosphere to a whole 
atmosphere, and yielded throughout a perfectly measurable action. The 
absorption by this gas, under the pressure of an atmosphere, amounted to fully 
81 per cent, of the incident radiation. 

Experiments on sulphuric-ether vapour are also recorded in which the pres¬ 
sures employed varied from 50 ^ 000 ^ °f an atmosphere to the maximum pressure 
of the vapour, the quantity of vapour corresponding to the smallest of these 
pressures being proved capable of producing a measurable effect. Comparing 
equal pressures up to 5 inches of mercury, sulphuric-ether vapour was found to 
be more than twice as potent as olefiant gas. 

The action of the following vapours at various degrees of density upon 
radiant heat is afterwards recorded :—Bisulphide of carbon, amylene, iodide of 
ethyl, iodide of methyl, iodide of amyl, chloride of amyl, benzol, methylic 
alcohol, formic ether, propionic ether, chloroform, and alcohol. 

A curious explosive effect occurring in the cylinders of the air-pump when 
air is mixed with attenuated bisulpkide-of-carbon vapour is referred to and 
explained. 

Chlorine gas is next operated on, and its action in altering the reflective 
power of the interior surface of the experimental tube is demonstrated. This 



4 


ANALYSIS OF MEMOIR I. 


leads to a searching inquiry whether any of the effects observed with the vapours 
above mentioned could be due to a diminution of interior reflexion. Two 
parallel series of experiments executed with two different experimental tubes, 
the one polished, and the other blackened within, are recorded. In nine cases 
out of thirteen the order of absorption in both tubes was the same; in fact, the 
absorptions in the blackened tube multiplied by a certain coefficient were 
sensibly equal to those in the polished tube. In the case ot the four remaining 
vapours slight deviations from the order of absorption were observed. These 
it was deemed unnecessary to follow up. so conclusive teas the evidence that the 
observed effects were really cases of absorption , and by no means to be referred to 
any alteration of the refecting surface whether by chemical action or by condensa¬ 
tion. 

The actions of other permanent gases than those already referred to are 
then recorded. 

Experiments are next described which illustrate the action of the aqueous 
vapour of our atmosphere on radiant heat; and considerations follow regarding 
the influence of an atmosphere like ours upon the temperature of a planet. In 
former speculations upon this subject the density and height of the atmosphere 
were dwelt upon by distinguished writers; but it is here pointed out that a 
comparatively slight change in the variable constituents of our atmosphere, by 
permitting free access of solar heat to the earth, and checking the outflow of 
terrestrial heat towards space, would produce changes of climate as great as 
those which the discoveries of geology reveal. 

Thus far our attention has been restricted to the absorption of radiant heat by 
gaseous matter, not only the general fact of absorption-, but vast differences of 
absorptive power being established experimentally. We now come to a series of 
reciprocal experiments on the radiation of heat by gases, which demonstrate 
not only the general fact of radiation, but that the order of radiation is precisely 
the same as the order of absorption. As regards both radiation and absorption 
the elementary gases in the experiments here recorded, stand lowest; olefiant 
gas highest; and between these extremes stand the other compound gases 
without any shifting of position. 

It is further shown that a film of gas, coating a polished metallic surface, may, 
both as regards radiation and absorption, be made to do the duty of a coat of 
varnish, or of lampblack, in increasing the emissive and absorptive power of 
the surface. 

Our knowledge of this subject prior to the foregoing experiments is thus 
briefly summed up by Melloni. ‘On ne commit encore aucun fait qui demontre 
directement le pouvoir emissif des fluides elastiques purs et transparents.’— 
Armales de Chimie et de Physique, vol. xxii. p. 494. 

Directly bearing upon this portion of the subject is an observation which, for 
a time, constituted one of the numerous perplexities besetting this inquiry in 
its earlier stages. A residue of vapour being in the experimental tube, air is 
permitted to enter; a prompt deflection follows, indicating an increase instead of 
a diminution of the transmission. The needle then returns, and finally, takes 
up a position indicating a slightly higher absorption than when the vapour- 
residue alone was present. On pumping out, the needle at first moves promptly, 
indicating a diminution instead of an increase of the transmission. After a 
momentary impulse in this direction, the needle returns to zero. The observation 
on analysis turned out to be an exceedingly interesting case of gaseous radiation 


ANALYSIS OF MEMOIR I. 


5 


and absorption. TV lien the air entered, the vapour was dynamically heated ; 
it discharged its heat against the pile, and thus apparently augmented the 
transmission. When the tube was exhausted the vapour was chilled, and the 
radiation into it from the adjacent face of the pile produced for a moment the 
deflection due to absorption. In subsequent memoirs, under the name of 
Dynamic Radiation and Absorption, this subject is fully developed. 

In the last section of the memoir an attempt is made to establish a physical 
connexion between radiation, absorption, and conduction. One of the specu¬ 
lative notions in this section subsequent experience has caused me to modify. 
Radiation and Absorption are here regarded as the acts of the molecule as a 
whole, whereas I now hold them to be mainly the work of the constituent 
atoms of the molecule. Experimental reasons for this change of conception 
will be given subsequently. The ^memoir winds up with some supplementary 
remarks on the thermo-electric pile and galvanometer, intended chiefly for the 
use of the younger student. 





% 


" 





























ON THE ABSORPTION AND RADIATION OF HEAT BY 
GASES AND VAPOURS, AND ON THE PHYSICAL CON¬ 
NEXION OF RADIATION, ABSORPTION, AND CON¬ 
DUCTION. 


The Balcerian Lecture delivered before the Royal Society , 

February 7, 1861.* 


INTRODUCTION. 

\ 

The researches on Glaciers winch. I have had the honour 
of submitting from time to time to the Royal Society 
directed my attention in a special manner to the observations 
and speculations of De Saussure, Fourier, Pouillet, and 
Hopkins, on the transmission of solar and terrestrial heat 
through the earth’s atmosphere, and gave practical effect to 
a desire long previously entertained to make the mutual 
action of radiant heat and gases and vapours of all kinds the 
subject of experimental inquiry. 

Our acquaintance with this department of Physics is exceed¬ 
ingly limited. So far as my knowledge extends, the literature 
of the subject may be stated in a few words. 

From experiments with his admirable thermo-electric appa¬ 
ratus, Melloni inferred that for a distance of 18 or 20 feet the 
absorption of radiant heat by atmospheric air is perfectly insen¬ 
sible.f 


* Received January 10. Philosophical Transactions for 1861 ; Philosophical 
Magazine , vol. xxii. p. 169. 
f La Thermochrosc, p. 136. 




8 


THE ABSORPTION AND RADIATION’ OF HEAT 


Witli a delicate apparatus of the same kind, Dr. Franz, of 
Berlin, found that the air contained in a tube 3 feet long 
absorbed 3’54 per cent, of the heat sent through it from an 
Argand lamp; that is to saj, calling the number of rajs which 
passed through the exhausted tube 100, the number which 
passed when the tube was filled with air was only 96*46.* 

In the sequel it will be shown that the result obtained bj 
Dr. Franz was due to an inadvertence in his mode of ob¬ 
servation. These are the only experiments of this nature with 
which I am acquainted, and they leave the field of inquiry 
now before us perfectly unbroken ground.f 

§ 1 . 

The Galvanometer and its Defects.—Magnetic Analysis of its Wire. 

At an early stage of the investigation I experienced the need 
of a first-class galvanometer. My instrument was constructed 
by that excellent workman Sauerwald, of Berlin. The needles 
are suspended independently of the shade, which is con¬ 
structed so as to enclose the smallest possible amount of air, 
the disturbance of aerial currents being thereby practically 
avoided. The plane glass plate, which forms the cover of the 
instrument, is close to the needle; so that the position of the 
latter can be read off with ease and accuracy either by the naked 
eye or by a magnifying lens. 

The wire of the coil belonging to this instrument was drawn 
from copper obtained from a galvano-plastic manufactory in the 
Prussian capital; but it was not free from magnetic action. 

In consequence of this, when the needles were as perfectly 
astatic as I could make them they deviated as much as 30° 
right and left of the neutral line. To neutralise this deflection, 
a minute magnetic ‘ compensator ’ was made use of, by which 
the needle was gently drawn to zero in opposition to the 
magnetism of the coil. 

But the instrument suffered much in point of delicacy from 
this arrangement, and accurate quantitative determinations 

* Pogg. Ann. vol. xciv. p. 342. 

f No doubt many experimenters had attempted to establish the action of air upon 
radiant heat; otherwise the conviction could not have become universal that no such 
action was discoverable. 


BY GASES AND YAPOUKS. 


9 


with, it were unattainable. I therefore sought to replace the 
Beilin coil by a less magnetic one. Mr. Becker first supplied 

me with a coil which reduced the lateral deflection from 30° 
to 3°. 

But e\en this small residue was a source of great annoy¬ 
ance, and for a time I almost despaired of obtaining pure 
copper wire. I knew that Professor Magnus had succeeded in 
obtaining it for his galvanometer, but the labour of doing so 
was immense. He first fused, and had drawn into wire, copper 
obtained from a galvano-plastic manufactory, but found, after 
the completion of his coil, its magnetic condition intolerable. 

I have therefore/ he says, ‘specially purified my copper in 
the following manner: A solution of sulphate of copper was 
saturated with ammonia, until the precipitated oxide was again 
dissolved. The precipitated oxide of iron was removed by filtra¬ 
tion, and, as copper is not easily precipitated eiectrolytically 
from an ammoniacal solution, the fluid was evaporated to 
dryness, and all the ammonia thus drawn off. The sulphate of 
copper thus purified .was dissolved in water and precipitated by 
the Voltaic current. As it was found impossible to separate 
the copper in an adherent mass, it was necessary to fuse it. 
Unhappily a very brittle metal is thus obtained, which cannot 
be drawn into wires. The metal had to be fused eight times 
in succession before it was rendered fit for this purpose. 
This process/ continues Magnus, c of purifying copper is very 
troublesome and very costly. Without doubt it would be 
possible to obtain silver quite as free from magnetism as this 
wire, and by an easy calculation it might be proved that it 
would cost considerably less than an equal weig’lit of copper 
prepared in the foregoing way.’ * 

* Pogg. Ann. vol. Ixxxiii. p. 489. 

Melloni gives the following account of the formidable nuisance of a magnetic 
coil: * Les systemes astatiques tres-sensibles appliques comme nous venons de l’indi- 
quer aux helices d’un fil ordinaire de cuivre ou d’argent, presentent presque toujours le 
fait curicux de ne pouvoir s’arreter au zero du cadran; c’est-a-dire que, generalement, 
les systemes astatiques doues dune grande sensibilite ne peuvent s’arreter dans le plan 
vertical qui divise l’helice en deux portions egales, parallelement a la direction des 
spires. Lorsqu'on cherche a les amener dans ce plan, en tournant doucement l'helice 
vers leur position d’equilibre, on les voit s’ecarter aussitot, a droite ou a gauche, et, 
apres quelqnes oscillations, se fixer stablement dans une position d’equilibre plus ou 
moins eloignee du zero. On mesure aisement cet arc de deviation moyennant un cercle 
gradue que l’on fixe a la partie superieure de l'helice apres y avoir pratique une ouver- 


10 


THE ABSORPTION AND RADIATION OF HEAT 


While pondering over the means of avoiding so formidable a 
task, the thought occurred to me that a magnet furnished an 
immediate and perfect test as to the quality of the wire. Pure 
copper is diamagnetic ; hence its repulsion or attraction by the 
m agnet would at once declare its fitness or unfitness for the 
purpose in view. 

Naked fragments of the wire furnished by M. Sauerwald 
were strongly attracted by the magnet. The wire furnished 
by Mr. Becker, when covered with its green silk, was also 
attracted, though in a much feebler degree. 

I then removed the silk covering from the latter and tested 
the naked wire. It was repelled. The whole annoyance in its 
case was thus fastened on the green silk; some iron compound 
had been used in the dyeing of it, and to this the deviation of 
the needle from zero was manifestly due. 

I had the green coating removed and the wire overspun 
with silk, clean hands being used in the process. A perfect 
galvanometer is the result. The needle, when released from the 
action of the current, returns accurately to zero, and is perfectly 
free from all magnetic action on the part of the coil. In fact, 
while we have been devising agate plates and other elaborate 
methods to gefc rid of the impurities of our galvanometer coils, 
the means of doing so by magnetic analysis are at hand. 
Diamagnetic copper wires are readily found. Out of eleven 
specimens, four of which were furnished by Mr. Becker, and 
seven taken at random from our laboratory, nine were found 
diamagnetic and only two paramagnetic. 

Perhaps the only defect of those noble instruments with which 

ture longituclinale dans le sens du zero et de la division des spires. La deviation est 
egale des deux cotes : elle peut aller jusqu'a 10 ou 12 degres et meme davantage, si, 
en operant sur un systeme astatique d’une grande perfection, on donne line certaine 
largcur a la fente qui sert d introduire dans I'helice laiguille inferieure du systeme. 
Le phenomena derive done du partage du fil en deux masses egales, qui ont 
chacune un centre d’attraction, vers lequel tendent les poles des aiguilles aimant^es. 
Ainsi le cuivre, dont ces fils sont orainairement composes, tout en n’etant pas un metal 
magnetique par lui-meme, opere sur les aiguilles aimantees comme s’il contenait des 
parcelles de fer. C’est, en effet, le cas da cuivre de commerce ; et l’on en devine facile- 
ment le motif, lorsqu’on reflechit a Timperfection des precedes de raffinago et au con¬ 
tact des outils employes dans les transformations successives du cuivre en rosettes, en 
verges et en fils. Et il ne faut pas s’imaginer que le fil ordinaire d’argent soit en de 
meilleures conditions; car l’argent, qui se trouve presque toujours en presence du fer 
pendant les operations necessaires a son extraction, ne se convertit en fil qu’a l’aide du 
marteau et des filieres d’acier.’ 


BY GASES AND VAPOURS. 


11 


Du Bois-Raymond conducts liis researches in animal electricity 
is that here alluded to. The needle never comes to zero, hut 
is drawn to it by a minute magnet. This defect may be com¬ 
pletely removed. By making sure at the outset that the naked 
wire is diamagnetic, and by the substitution of clean white silk 
for green, the compensator may be dispensed with and a per¬ 
fect instrument secured. It is never necessary to wait for the 
completion of the coil to test the quality of the wire.* 

§ 2 . 

First Experiments on Absorption by Ordinary Methods. 

Our present knowledge of the deportment of liquids and 
solids would lead to the inference that, if gases and vapours 
exercised any appreciable absorptive power on radiant heat, the 
absorption would make itself most manifest on heat emanating 
from an obscure source. But an experimental difficulty occurs 
at the outset in dealing with such heat. How must we close 
the chamber containing the gases through which the calorific 
rays are to be sent ? Melloni found that a glass plate one-tenth 
of an inch in thickness intercepted all the rays emanating from 
a source of the temperature of boiling water, and fully 94 per 
cent, of the heat from a source of 400° Centigrade. Hence a 
tube closed with glass plates would be scarcely more suitable for 
the purpose now under consideration than if its ends were 
stopped by plates of metal. 

Rock-salt immediately suggests itself as the proper substance; 
but to obtain plates of suitable size and transparency was ex¬ 
ceedingly difficult. Indeed, had I been less efficiently seconded, 
the obstacles thus arising might have been insuperable. To 
the Trustees of the British Museum I am indebted for the mate¬ 
rial of one good plate of salt; to Mr. Harlin for another ; while 
Mr. Lettsom, at the instance of Mr. Darker,f brought me a 
piece of salt from Germany from which two fair plates were 

* Mr. Becker, to whose skill and intelligence I have been greatly indebted, fur¬ 
nished me with several specimens of wire of the same fineness as that used by Du 
Bois-Raymond, some covered with green silk and others with white. The former 
were invariably attracted, the latter invariably repelled. In all cases the naked wire 
was repelled. 

f During the course of the inquiry I have often had occasion to avail myself of the 
assistance of this excellent mechanician. 


12 


THE ABSORPTION AND RADIATION OF HEAT 




taken. To Lady Murchison, Sir Emerson Tennent, Sir Philip 
Egerton, and Mr. Pattison my best thanks are also due for their 
friendly assistance. 

The first experiments were made with a tube of tin polished 
inside, 4 feet long and 2*4 inches in diameter, the ends of which 
were furnished with brass appendages to receive the plates of 
rock-salt. Each plate was pressed firmly against a flange by 
a bayonet joint, being separated from the flange by a suitable 
washer. Various descriptions of leather washers were tried for 
this purpose and rejected. The substance finally chosen was 
vulcanised indiarubber very lightly smeared with a mixture of 
bees’-wax and spermaceti. A T-piece was attached to the tube, 
communicating on one side with a good air-pump, and on the 
other with the external air, or with a vessel containing the gas 
to be examined. 

The tube being mounted horizontally, a Leslie’s tube contain¬ 
ing hot water was placed close to one of its ends, while an excel¬ 
lent thermo-electric pile, connected with its galvanometer, was 
presented to the other. The tube being exhausted, the calorific 
rays sent through it fell upon the pile, a permanent deflection of 
30° being the consequence. The temperature of the water was 
in the first instance purposely so arranged as to produce this 
deflection. 

Dry air was now admitted into the tube, while the needle of 
the galvanometer was observed with all possible care. Even by 
the aid of a magnifying lens I could not detect the slightest 
change of position. Oxygen, hydrogen, and nitrogen, subjected 
to the same test, gave the same negative result. The tempera¬ 
ture of the water was subsequently lowered, so as to produce a 
deflection of 20° and 10° in succession, and then heightened till 
the deflection amounted to 40°, 50°, 60°, and 70°; but in no 
case did the admission of air, or any of the above gases, into the 
exhausted tube produce any sensible change in the position of 
the needle. 

It is a well-known peculiarity of the galvanometer, that its 
higher and lower degrees represent different amounts of calorific 
action. In my instrument, for example, the quantity of heat 
necessary to move the needle from 70° to 71° is about twenty 
times that required to move it from 11° to 12°.* Now in the 

* See Remarks, 1872, at the end of this memoir. 


BY GASES AND VAPOURS. 


13 


case of the small deflections above referred to the needle was, it 
is true, in a sensitive position; but then the total amount of 
heat passing through the tube was so inconsiderable that a small 
percentage of it, even if absorbed, might well escape detection. 
In the case of the large deflections, on the other hand, a very 
considerable abstraction of heat would be necessary to produce 
any sensible diminution of the deflection. Hence arose the 
thought of operating, if possible, with large quantities of heat, 
while the needle intended to reveal its absorption should con¬ 
tinue to occupy its position of maximum delicacy. 

§ 3. 

Method of Compensation . 

The first attempt at solving this problem was as follows:— 
My galvanometer is a differential one—the coil being composed 
of two wires wound side by side, so that a current can be sent 
through either of them independent of the other. The thermo¬ 
electric pile was placed at one end of the tin tube, and the ends 
of one of the galvanometer wires were connected with it. A 
copper ball heated to low redness being placed at the other end 
of the tube, the needle of the galvanometer was propelled to its 
stops at 90°. The ends of the second wire were now so attached 
to a second pile that when the latter was caused to approach 
the copper ball, the current excited passed through the coil in 
a direction opposed to the first one. Gradually, as the second 
pile was brought nearer to the source of heat, the needle de¬ 
scended from the stops, and when the two currents were equal 
the position of the needle was at zero. 

Here, then, we had a powerful flux of heat through the tube ; 
and if a column of gas four feet long exercised any sensible 
absorption, the needle was in the position best calculated to 
reveal it. In the first experiment made in this way, the neutral¬ 
isation of one current by the other occurred when the tube was 
filled with air; on exhausting the tube, the needle started 
suddenly off m a direction which indicated that a less amount 
of heat passed through the partially exhausted tube than 
through the filled one. The needle, however, soon stopped, 
turned, descended quickly to zero, and passed on to the other 
side, where its deflection became permanent. The air employed 


14 


THE ABSORPTION AND RADIATION OF HEAT 


in this experiment came direct from the laboratory, and the first 
impulsion of the needle was probably due to the aqueous vapour 
precipitated as a cloud by the sudden exhaustion of the 
tube; for when, previous to its admission, the air passed over 
chloride of calcium, or pumice-stone moistened with sulphuric 
acid, no such effect was observed. The needle moved steadily in 
one direction till its maximum deflection was attained, and 
this deflection showed that in all cases radiant heat was absorbed 
by the air within the tube. 

These experiments were begun in the spring of 1859, and 
continued without intermission for seven weeks. The course 
of the inquiry during this whole period was an incessant struggle 
with experimental difficulties. Approximate results were easily 
obtainable ; but I aimed at exact measurements, which could not 
be made with a varying source of heat like the copper ball. To 
obtain a high and steady source of heat I resorted to copper 
cubes containing fusible metal, or oil, but was not satisfied 
with their action. Finally, a lamp was constructed which 
poured a steady sheet of flame against a plate of copper ; and, 
to keep the flame constant, a gas regulator specially constructed 
for me by Mr. Hulet was made use of. It was also arranged 
that the radiating plate should form one of the walls of a 
chamber which could be connected.with the air-pump and ex¬ 
hausted, so that the heat emitted by the copper plate might 
cross a vacuum before entering the experimental tube. With 
this apparatus, during the summer of 1859, I approximately 
determined the absorption of nine gases and twenty vapours. 
The results would furnish materials for a Ions; memoir; but 
increased experience and improved methods have enabled me to 
substitute for those results others of greater accuracy; I shall 
therefore pass over the work of these seven weeks without 
further allusion to it. 

On September 9 of the present year (1860) the inquiry was 
resumed. For three weeks the heated plate of copper was 
my source of heat, but it was finally rejected on the score 
of insufficient constancy. The cube of hot oil was again 
resorted to, and with it I continued to work up to Monday, 
October 29. During these seven weeks, from eight to ten 
hours daily were devoted to experiments; but the results, 
though more accurate, must unhappily share the fate of those 


BY GASES AND VAPOURS. 


15 


obtained in 1859. In fact, these fourteen weeks of labour con¬ 
stituted a period of discipline, during which a continued 
struggle was carried on against the difficulties of the subject 
and the defects of the locality in which the inquiry was con¬ 
ducted. 

My reason for trying these high sources of heat was this : 
the absorptive power of some of the gases examined was so 
small that, to make it clearly evident, a powerful beam of radiant 
heat was essential. For other gases, and for all the vapours 
that had come under mv notice, sources of lower temperature 
would have been not only sufficient, but far preferable. I was 
finally induced to resort to boiling water, which, though it gave 
greatly diminished effects, was capable of being preserved at so 
constant a temperature that deflections which, with the other 
sources, would be disturbed by errors of observation, became 
with it true quantitative measures of absorption. 

§4. 

Final Form of Apparatus. 

The entire apparatus made use of in the experiments on 
. absorption is figured on the Frontispiece. S S' is the experi¬ 
mental tube , composed of brass, polished within, and connected, 
as shown in the figure, with the air-pump A A. At S and S' 
are the plates of rock-salt which close the tube air-tight. 
The length from S to S' is 4 feet. C is a cube, containing 
boiling water, in which is immersed the thermometer t. The 
cube is of cast-copper, and on one of its faces was a projecting 
ring, to which a brass tube of the same diameter as S S', and 
capable of being connected air-tight with the latter, was carefully 
soldered. The face of the cube within the ring is the radiating 
plate, which is coated with lampblack. Thus between the cube 
C and the first plate of rock-salt there is a front chamber F, con¬ 
nected with the air-pump of the flexible tube D D, and capable 
of being exhausted independently of S S'. To prevent the 
heat of conduction from reaching the plate of rock-salt S, the 
tube F is caused to pass through a vessel V, being soldered 
to the latter where it enters it and issues from it. This vessel 
is supplied with a continuous flow of cold water through the 
influx tube ii, which dips to the bottom of the vessel;* the 



16 THE ABSORPTION AND RADIATION OF HEAT 

water escapes through the efflux tube e e, and the continued 
circulation of the cold liquid completely intercepts the heat 

that would otherwise reach the plate S. 

The cube C is heated by the gas-lamp L. P is the thermo¬ 
electric pile placed on its stand at the end of the experimental 
tube, and furnished with two conical reflectors, as shown in the 
figure. C' is the compensating cube, used to neutralise by its 
radiation * the effect of the rays passing through S S'. The 
regulation of this neutralisation was an operation of some 
delicacy; to effect it the double screen H was connected with 
a winch and screw arrangement, by which it could be advanced 
or withdrawn through extremely minute spaces. For this most 
useful adjunct I am indebted to the kindness of my friend, 
Mr. Gassiot. N N is the galvanometer, with perfectly astatic 
needles and perfectly non-magnetic coil ; it is connected with 
the pile P by the wires w w ; Y Y is a system of six chloride 
of calcium tubes, each 32 inches long; P is a U-tube, containing 
fragments of pumice-stone, moistened with strong caustic 
potash; and Z is a second similar tube, containing fragments 
of pumice-stone wetted with strong sulphuric acid. When 
drying only was aimed at, the potash tube was suppressed. 
When, on the contrary, as in the case of atmospheric air, both 
moisture and carbonic acid were to be removed, the potash tube 
was included. G G is a holder from which the gas to be ex¬ 
perimented with was sent through the drying tubes, and thence 
through the pipe pp into the experimental tube S S'. The 
appendage at M and the arrangement at 0 0 may for the 
present be disregarded; I shall refer to them particularly 
by-and-by. 

The mode of proceeding was as follows: The tube S S' and 
the chamber F being exhausted as perfectly as possible, the 
connexion between them was intercepted by shutting off the 
cocks m in'. The rays from the interior blackened surface of 
the cube C passed first across the vacuum F, then through the 
plate of rock-salt S, traversed the experimental tube, crossed 
the second plate S', and being concentrated by the anterior 
conical reflector, impinged upon the adjacent face of the pile P. 

* It -will be seen that in this arrangement I have abandoned the use of the 
differential galvanometer, and made the thermo-electric pile itself the differential 
instrument. 


BY GASES AND VAPOURS. 


17 


Meanwhile the rays from the hot cube Cf fell upon the opposite 
face of the pile, and the position of the galvanometer needle 
declared at once which source was predominant. A movement 
of the screen H back or forward with the hand sufficed to 
establish an approximate equality; but to make the radiations 
perfectly equal, and thus bring the needle exactly to 0°, the 
fine motion of the screw above referred to was necessary. 

The needle being at 0°, the gas to be examined was admitted 
into the tube, passing, in the first place, through the drying 
apparatus. Any required quantity of the gas might be admitted ; 
and here experiments on gases and vapours enjoy an advantage 
over those with liquids and solids—namely, the capability of 
changing the density at pleasure. When the required quantity 
of gas had been admitted the galvanometer was observed, and 
from the deflection of its needle the absorption was accurately 
determined. 

The galvanometer was calibrated by the method recommended 
by Melloni Tliermochrose,’ p. 59), the precise value of its larger 
deflections being at once obtained by reference to a table. Up 
to the 30th degree, or thereabouts, the deflections may be 
regarded as the expression of the absorption; bnt beyond this 
the absorption equivalent to any deflection was obtained from 
the table of calibration. 


§ 5 . 

ABSORPTION OF RADIANT HEAT. 

First Results.—Action of Ozone and of Compound Gases on 

Radiant Heat. 

The air of the laboratory, freed from its moisture and 
carbonic acid, and permitted to enter until the tube 
was filled, produced a deflection of about . . . 1° 

Oxygen obtained from chlorate of potash and peroxide of 
manganese produced a deflection of about ... 1 

One specimen of nitrogen, obtained from the decomposi¬ 
tion of nitrate of potash, produced a deflection of about 1° 
Hydrogen from zinc and sulphuric acid produced a deflec¬ 
tion of about ........ 1 

Hydrogen obtained from the electrolysis of water produced 

a deflection of about ....... 1° 


2 


18 


THE ABSORPTION AND RADIATION OF HEAT 


Oxygen obtained from tlie electrolysis of water, and sent 
through, a series of eight bulbs containing a strong 
solution of iodide of potassium, produced a deflection of 

about.1 

In the last experiment the electrolytic oxygen was freed 
from its ozone. The iodide of potassium was afterwards 
suppressed, and the oxygen, plus its ozone, admitted 
into the tube; the deflection produced was ... 4° 

Hence the small quantity of ozone which accompanied the 
oxygen in this case trebled the absorption of the oxygen itself.* 
I have repeated this experiment many times, employing 
different sources of heat. With sources of high temperature 
the difference between the ozone and the ordinary oxygen comes 
out very strikingly. By careful decomposition a much larger 
amount of ozone might be obtained, and a correspondingly large 
effect on radiant heat. 

In obtaining the electrolytic oxygen two different vessels were 
made use of. To diminish the resistance of the acidulated 
water to the passage of the current, I placed in one vessel a 
pair of very large platinum plates, between which the current 
from a battery of ten Grove’s cells was transmitted. The 
oxygen bubbles liberated were extremely minute, and the gas, 
on being sent through iodide of potassium, scarcely coloured 
the liquid; the characteristic odour of ozone was almost entirely 
absent, and there was little or no action upon radiant heat. In 
the second vessel smaller plates were used. The bubbles_ of 
oxygen were much larger, and did not come into such intimate 
contact with either the platinum or the water. The oxygen 
thus obtained showed the characteristic reactions of ozone; 
and with it the above result was obtained. 

The total amount of heat transmitted through the tube in 

* \' t 

these experiments produced a deflection of . V .. . 71*5° 

Taking as unit of heat the quantity necessary to cause the 
needle to move from 0° to 1°, the number of units ex¬ 
pressed by the above deflection is .... 308 

Hence the absorption by the above gases amounted to about 
0*33 per cent. 

* It will be shown subsequently that this result is in harmony with the supposition 
that ozone is a compound body. See Memoir II. §§ 17, 18, and 19. 


BY GASES AND VAPOURS. 


19 


I am unable at the present moment to range with certainty 
oxygen, hydrogen, nitrogen, and atmospheric air in the order 
of their absorptive powers, though several hundred experiments 
have been made with the view of doing so. The proper action of 
these gases is so small that the slightest foreign impurity gives 
one a predominance over the other. In preparing the gases 
the methods recommended in chemical treatises have been re¬ 
sorted to, but as yet only to discover the defects incidental to 
these methods. Augmented experience and the assistance of 
my friends will, I trust, enable me to solve this point by-and- 
by. An examination of the whole of the experiments induces 
me to regard hydrogen as the gas which exercises the lowest 
absorptive power.* 

We have here the cases of minimum gaseous absorption. It 
will be interesting to place in juxtaposition with the above 
results some of those obtained with olefiant gas—the most 
highly absorbent permanent gas that I have hitherto examined. 
I select for this purpose an experiment made on November 21. 

The needle being steady at zero in consequence of the 
equality of the actions on the opposite faces of the pile, 
the admission of olefiant gas gave a permanent deflec¬ 
tion of.70*3° 

The gas being completely removed, and the equilibrium 
re-established, a plate of polished metal was interposed 
between one of the faces of the poile and the source 
of heat adjacent. The total amount of heat passing 
through the exhausted tube was thus found to produce 
a deflection of ....... 75° 

Now a deflection of 70*3° is equivalent to 290 units, and a 
deflection of 75° is equivalent to 360 units ; hence more than 
seven-ninths of the total heat, about 81 per cent., were cut off 
by the olefiant gas. 


* The test to which these gases were subjected was far more severe than any pre¬ 
viously applied, but the result was in practical accordance with the conviction 
that, at all events, in transparent gases, the absorption of radiant heat, if not absolutely 
insensible, was, at all events, beyond the reach of experiment. But early in 1859, 
coal-gas being at hand, I tried it, and found its deportment more like that of an 
adiathermic solid than that of a gas. A crowd of other gases was immediately 
added. In fact, this single experiment with coal-gas opened the door to all the 
researches recorded in this volume. [1872.] 


20 THE ABSORPTION AND RADIATION OF HEAT 

The extraordinary energy with which the needle was deflected 
when the olefiant gas was admitted into the tube, was such as 
might occur had the plates of rock-salt become suddenly covered 
with something opaque. To test whether any such action 
occurred, I carefully polished a plate, and against it was pro¬ 
jected for a considerable time a stream of the gas; there was no 
dimness produced. The plates of rock-salt, moreover, which 
were removed daily from the tube, usually appeared as bright 

when taken out as when they were put in.* 

The gas in these experiments issued from its holder, where it 
had been in contact with cold water. To test whether it had 
so chilled the plates of rock-salt as to produce the effect, a 
similar holder was filled with atmospheric .air and permitted it 
to attain the temperature of the water; but the action of the 
air was not thereby sensibly augmented. 

In order to subject the gas to ocular examination, I had a 
£'lass tube constructed and connected with the air-pump. On 
permitting olefiant gas to enter it not the slightest dimness 01 
opacity was observed. To remove the last trace of doubt as to 
the possible action of the gas on the plates of rock-salt, 
the tin tube referred to at page 12 was perforated at its 
centre and a cock inserted into it 5 the source of heat was at 
one end of the tube, and the thermo-electric pile at some 
distance from the other. The plates of salt were entirely aban¬ 
doned, the tube being open at its ends and consequently full of 
air. On allowing the olefiant gas to stream for a second or 
two into the tube through the central cock, the needle flew off 
and struck against its stops. It was held steadily for a con¬ 
siderable time between 80° and 90°. 

A slow current of air sent through the tube gradually re¬ 
moved the gas, and the needle returned accurately to zero. 

The gas within the holder being under a pressure of about 
twelve inches of water, the cock attached to the cube was 
turned quickly on and off; the quantity of gas which entered 
the tube in this brief interval was sufficient to cause the needle 
to be driven to the stops, and steadily held between 60° and 70°. 

The gas being again removed, the cock was turned once half 
round as quickly as possible. The needle was driven in the 

* From the very beginning of the inquiry my attention was awake to the possibility 
of precipitation upon the plates of rock-salt. [1872.] 


BY GASES AND VAPOURS. 


21 


first instance through an arc of 60°, and was held permanently 
at 50°. 

The quantity of gas which produced this last effect, on being 
admitted into a graduated tube, was found not to exceed one- 
sixth of a cubic inch in volume. 

The tin tube was now taken away, and both sources of heat 
allowed to act from some distance on the thermo-electric pile. 
When the needle was at zero, olefiant gas was allowed to issue 
from a common Argand burner into the air between one of the 
sources of heat and the pile. The gas was invisible—nothing 
was seen in the air—but the needle immediately declared its pre¬ 
sence, being driven through an arc of 41°. In the four experi¬ 
ments last described the source of heat was a cube of oil heated 
to 250° Centigrade, the compensation cube being filled with 
boiling water.* 

Those who, like myself, have been taught to regard trans¬ 
parent gases as sensibly diatliermanous, will probably share the 
astonishment with which I witnessed the foregoing effects. I 
was, indeed, slow to believe it possible that a body so constituted, 
and so transparent to light as olefiant gas, could be so densely 
opaque to any kind of rays ; and, to secure myself against 
error, several hundred experiments were executed with this 
single substance. But the citing of them at greater length 
could not add to the conclusiveness of the proofs just furnished, 
that the case is one of true calorific absorption.f 

§ 6 . 

Variations of Density.—Relation of Absorption to Quantity 

of Matter. 

Having thus established in a general way the absorptive 
power of olefiant gas, the question arises, What is the relation 
which subsists between the density of the gas and the quantity 
of heat extinguished ? 

* With a cube containing boiling water this experiment has been since made visible 
to a large assembly. 

f It is evident that the old mode of experiment might be applied to this gas. In¬ 
deed, several of the solids examined by Melloni are inferior to it in absorptive power. 
Had time permitted, I should have checked my results by experiments made in the 
usual way; this, however, will be done on a future occasion. 


22 THE ABSORPTION AND RADIATION OF HEAT 

I souglit at first to answer this question in the following 
way : An ordinary mercurial gauge was attached to the air- 
pump. The experimental tube being exhausted, and the needle 
of the galvanometer at zero, olefiant gas was admitted until it 
depressed the mercurial column 1 inch, the consequent de¬ 
flection being noted. The gas was then admitted until a de¬ 
pression of 2 inches was observed, and thus the absorption 
effected by gas of 1, 2, 3, and more inches’ pressure was deter¬ 
mined. In the following table the first column contains the 
pressures in inches of mercury, the second the deflections, and 
the third the absorption equivalent to each deflection. 




Table I .—Olefiant Gas. 



Pressures in 
inches 

Deflections 

O 

Absorption 
per 100 

Pressures in 
inches 

Deflections 

O 

Absorption 
per 100 

1 

56 

90 

7 

61-4 

182 

2 

58-2 

123 

8 

61-7 

186 

o 

O 

59-3 

142 

9 

62 

190 

4 

60 

157 

10 

62-2 

192 

5 

60-5 

168 

20 

66 

227 

6 

61 

177 




IsTo definite relation between the density of the gas 

and its 


absorption is here exhibited. We see that an augmentation of 
the density seven times about doubles the amount of the absorp¬ 
tion ; while gas of 20 inches’ pressure effects only 2 \ times the 
absorption of gas under 1 inch of pressure. 

But here the following reflections suggest themselves : It is 
evident that olefiant gas of 1 inch pressure, producing so large a 
deflection as 56°, must extinguish a large proportion of the rays 
which are capable of being absorbed by the gas, and hence the 
succeeding measures, having a less and less amount of heat to 
act upon, must produce a continually smaller effect. But sup¬ 
posing the quantity of gas first introduced to be so inconsider¬ 
able that the number of rays extinguished by it is a vanishing 
quantity compared with the total number capable of absorption, 
we might reasonably expect that in this case a double quantity 
of gas would produce a double effect, a treble quantity a treble 
effect, or, in general terms, that the absorption would, for a time, 
be proportional to the density. 

To test this idea a portion of the apparatus, purposely 
omitted in the description already given, was made use of. 



BY GASES AIs T D YAPOUKS. 


23 


0 0 (see Frontispiece) is a graduated glass tube, the end of which 
dips into the basin of water B. The tube can be closed above 
by means of the stopcock r ; d d is a tube containing fragments 
of chloride of calcium. The tube 0 O is first filled with water 
to the cock r ; this water is then displaced by olefiant gas; 
and afterwards the tube S S', and the entire space between the 
cock r and the experimental tube, is exhausted. The cock n 
being now closed and r' left open, the cock r at the top of the 
tube 0 0 is carefully turned on and the gas permitted to 
enter the tube S S' with extreme slowness. The wai^r rises in 
0 0, each of the smallest divisions of which represents a 
volume of ^th of a cubic inch. Successive measures of this 
capacity were admitted into the tube, the absorption in each 
case being determined. 

In the following table the first column expresses the quantity 
of gas admitted into the experimental tube, and the second 
the corresponding deflection, which, within the limits of the 
table, expresses the absorption; the third column contains the 
absorption, calculated on the supposition that it is proportional 
to the density. 

Table II.— Olefiant Gas. 

(Unit-measure ~th of a cubic inch.) 


Measures 
of Gas 

Absorption per 100 

Measures 
of Ga3 

Absorption per 100 

r 

Observed 

Calculated 

r 

Observed 

Calculated 

1 

2-2 

2-2 

9 

19 8 

198 

2 

45 

4-4 

10 

22 

22 

3 

6-6 

6-6 

11 

24 

24-2 

4 

88 

8-8 

12 

25-4 

26.4 

5 

11 

11 

13 

29 

28-6 

6 

12 

13-2 

14 

30-2 

29*8 

7 

148 

15*4 

15 

33-5 

33 

8 

16-8 

17.6 





This table shows the correctness of the foregoing surmise, and 
proves that for small quantities of gas the absorption is exactly 
proportional to the density. 

Let us pause for a moment to estimate the tenuity of the gas 
with which we have here operated. The length of the experi¬ 
mental tube is 48 inches, and its diameter 2*4 inches ; its volume 
is therefore 218 cubic inches. Adding to this the contents of 
the cocks and other conduits leading to the tube, we may assume 
that each fiftieth of a cubic inch of the gas had to diffuse itself 





24 


THE ABSORPTION AND RADIATION OF IIEAT 


through a space of 220 cubic inches. The pressure, therefore, of 
a single measure of the gas thus diffused would be ttoTo^ °f 
an atmosphere,—a pressure capable of depressing the mercurial 
column connected with the pump -j-^yth of an inch, or about 
•^jtli of a millimetre ! 



Action of Sulphuric-ether Vapour on Radiant Heat. 


But the absorptive energy of olefiant gas, extraordinary as it 
is shown to be by the above experiments, is far exceeded by that 
of some of the vapours of volatile liquids. A glass flask was 
provided with a brass cap furnished with an interior thread, by 
means of which a stopcock could be screwed airtight to the flask. 
Sulphuric ether being placed in the latter, the space above the 
liquid and the liquid itself were completely freed of air by 
means of an air-pump. The flask, with its closed stopcock, being 
attached to the experimental tube; the latter was exhausted 
and the needle brought to zero. The cock was then turned on 
so that the ether-vapour slowly entered the experimental tube. 
An assistant observed the gauge of the air-pump, and when it 
had sunk an inch, the stopcock was promptly closed. The con¬ 
sequent galvanometric deflection was then noted; a second 
quantity of the vapour, sufficient to depress the gauge another 
inch, was then admitted, and in this way the absorptions of 
five successive measures, each possessing within the tube 1 
inch of pressure, were determined. 

In the following table the first column contains the pressures 
in inches, the second the deflection due to each, and the third the 
amount of heat absorbed, expressed in the units already referred 
to. For the purpose of comparison the corresponding absorp¬ 
tions of olefiant gas are placed in the fourth column. 


Table III .—Sulphuric Ether. 


essures in 
inches 

Deflections 

Absorption 
per 100 

Corresponding Absorption 
by Olefiant Gas 

1 

64-8 

214 

90 

2 

70 

282 

123 

3 

72 

315 

142 

4 

73 

330 

154 

5 

73 

330 

163 




BY GASES AND VAPOURS. 


25 


For these pressures the absorption of radiant heat by the 
vapour of sulphuric ether is more than twice that of olefiant 
gas. We also observe that in the case of the vapour the 
successive absorptions approximate more quickly to equality. 
In fact, the absorption produced by 4 inches of the vapour is 
sensibly the same as that produced by 5. 

But reflections similar to those already applied to olefiant 
gas are also applicable to ether. Supposing we make our 
unit-measure small enough, the number of rays first de¬ 
stroyed will vanish in comparison with the total number, and 
for a time the fact will probably manifest itself that the absorp¬ 
tion is directly proportional to the density. To examine 
whether this is the case, a portion of the apparatus, omitted 
in the general description, was made use of. K is a small 
flask, with a brass cap, which is closely screwed to the stop¬ 
cock d. Between the cocks d and c, the latter connected 
with the experimental tube, is the chamber M, the capacity 
of which was accurately determined. The flask K being par¬ 
tially filled with ether, the air above the liquid is removed. 
The stopcock d being shut off and c turned on, the tube S S' 
and the chamber M are exhausted. The cock c being now 
shut off, and d turned on, the chamber M is filled with pure 
ether-vapour. By turning d off and c on, this vapour is 
allowed to diffuse itself through the experimental tube; suc¬ 
cessive measures are thus introduced, and the effect produced 
by each is noted. Measures of various capacities were made 
use of, according to the requirements of the vapours examined. 

In the first series of experiments made with this apparatus, I 
omitted to remove the air from the space above the liquid; each 
measure therefore sent in to the tube was a mixture of vapour 
and air. This diminished the effect of the vapour; but the 
proportionality, for small quantities, of density to absorption 
exhibits itself so decidedly as to induce me to record the obser¬ 
vations. The first column, as usual, contains the measures of 
vapour, the second the observed absorption, and the third 
the calculated absorption. The galvanometric deflections are 
omitted, their values being contained in the second column. 
In fact, as far as the seventh observation, the absorptions are 
merely the record of the deflections. 


26 


THE ABSORPTION AND RADIATION OF HEAT 


Table IY. —Mixture of Ether Vapour and Air. 

(Unit-measure g 5 th of a cubic inch.) 



Absorption per 100 


Absorption per 100 

■A - 

Measures 

r~ 

Observed 

—- s 

Calculated 

Measures 

Observed 

Calculated 

1 

4 o 

4\5 

21 

82-8 

95 

2 

9-2 

9 

22 

84 

99 

3 

13-5 

13-5 

23 

87 

104 

4 

18 

18 

24 

88 

108 

5 

22-8 

23-5 

25 

90 

113 

6 

27 

27 

26 

93 

117 

7 

31-8 

31-5 

27 

94 

122 

8 

36 

36 

28 

95 

126 

9 

397 

40 

29 

98 

131 

10 

45 

45 

30 

100 

135 

20 

81 

90 





Up to the 10th measure we find that ’density and absorption 
augment in precisely the same ratio. While the former varies 
from 1 to 10, the latter varies from 4‘5 to 45. At the 20th 
measure, however, a deviation from proportionality is apparent, 
and the divergence gradually augments from 20 to 30. In fact 
20 measures tell upon the heat capable of being absorbed—the 
quantity quenched becoming so considerable that at length every 
additional measure encounters a materially enfeebled beam, 
and hence produces a diminished effect. 

With ether vapour alone, the results recorded in the following 
table were obtained; and as I wished to know how far the 
pressure of the vapour might be diminished, the capacity of the 
unit-measure was reduced to T Uoth of a cubic inch. 

Table Y.— Sulphuric Ether. 


(Unit-measure -j^th of a cubic inch.) 



Absorption per 100 


Absorption per 100 

A 

Measures 

r 

Observed 

Calculated 

Measures 

r 

Observed 

Calculated 

1 

5 

4-6 

17 

65*5 

77-2 

2 

10-3 

9-2 

18 

68 

83 

4 

19-2 

18-4 

19 

70 

87-4 

5 

24-5 

23 

20 

72 

92 

6 

29-5 

27 

21 

73 

967 

7 

34*5 

32-2 

22 

73 

101-2 

8 

38 

368 

23 

73 

105-8 

9 

44 

41-4 

24 

77 

110-4 

10 

46*2 

46-2 

25 

78 

115 

11 

50 

50*6 

26 

78 

119-6 

12 

52-8 

55*2 

27 

80 

124-2 

13 

55 

598 

28 

80-5 

128-8 

14 

57-2 

64-4 

29 

81 

133-4 

15 

594 

69 

30 

81 

138 

16 

62*5 

73-6 













BY GASES AND VAPOUKS. 


27 


We here find the proportion between density and absorption 
sensibly preserved for the first eleven measures, after which 
the deviation gradually augments. Some specimens of ether 
have been examined which acted still more energetically on the 
thermal rays than that just referred to. 

No doubt for smaller measures than yAyth of a cubic inch 
the above law holds still more rigidly true; and in a suitable 
locality it would be easy to determine with perfect accuracy 
T yfch of the absorption produced by our first measure; this 
would correspond to yA—th of a cubic inch of vapour. But on 
entering the tube the vapour has only the tension due to the 
temperature of the laboratory, namely 12 inches. This would 
require to be multiplied by 2*5 to bring it up to that of the 
atmosphere. Hence the yyVoth of a cubic inch, the absorption 
of which has been affirmed cajoable of measurement, would, on 
being diffused through a tube possessing a capacity of 220 
cubic inches, have a pressure of —J— x A - x —A— = -— ]L ——th 
of an atmosphere! 

§ 8 . 

Extension of Inquiry to other Vapours. 

I have now to record the results obtained with thirteen other 
vapours. The method of experiment was in all cases the same 
as that employed in the case of ether, the only variable 
element being the size of the unit-measure. For with many 
substances no sensible effect could be obtained with the unit 
volume employed in the experiments last recorded. With 
bisulphide of carbon, for example, it was necessary to aug¬ 
ment the unit-measure 50 times to render the measurements 
satisfactory. 


Table YI .—Bisulphide of Carbon. 



(Unit-measure 
Absorption per 100 

i a cubic inch.) 

Absorption per 100 

A- 


fT 

\ 


r 

A 

Measures 

Observed 

Calculated 

Measures 

Observed 

Calculated 

1 

2-2 

2-2 

11 

16-2 

24-2 

2 

4-9 

44 

12 

16*8 

26-4 

3 

65 

66 

13 

17-5 

286 

4 

8-8 

8-8 

14 

18-2 

308 

5 

10-7 

11 

15 

19 

33 

6 

12-5 

13 

16 

20 

352 

7 

138 

15-4 

17 

20 

37-4 

8 

14-5 

17-6 

18 

20-2 

39-6 

9 

15 

19 

19 

21 

41-8 

10 

15*6 

22 

20 

21 

44 









28 


THE ABSORPTION AND RADIATION OF HEAT 


As far as tlie sixth measure the absorption is proportional to 
the density; after which the effect of each successive measure 
diminishes. Comparing the absorption effected by a quantity 
of vapour which depressed the mercury column half an inch, 
with that effected by vapour of one inch j)ressure, the same 
deviation from proportionality is observed. Thus :— 

By mercurial gauge. 

Pressure Absorption per 100 

£ inch 14‘8 

1 inch 18*8 

These numbers simply express the galvanometric deflections, 
which, as already stated, are strictly proportional to the absorp¬ 
tion as far as 30° or thereabouts. Did the law of pnyportion 
hold good, the absorption due to 1 inch of tension ought of 
course to be 29*6 instead of 18*8. 

Whether for equal volumes of the vapours at their maximum 
density, or for equal pressures as measured by the depression of 
the mercurial column, bisulphide of carbon exercises the lowest 
absorptive power of all the vapours hitherto examined. For 
very small quantities, a volume of sulphuric-ether vapour, at its 
maximum density in the unit-measure, and expanded thence 
into the experimental tube, absorbs 100 times the quantity of 
heat intercepted by an equal volume of bisulphide of carbon 
vapour at its maximum density. These substances mark the 
extreme limits of the scale, as far as my inquiries have hitherto 
proceeded. The action of every other vapour is less than that of 
sulphuric ether, and greater than that of bisulphide of carbon. 


Remarks on the Explosion of Bisulphide-of--Carbon Vapour in the 

Air-pump Cylinders. 

A very singular phenomenon was repeatedly observed during 
the experiments with bisulphide of carbon. After determining 
the absorption of the vapour, the tube was exhausted as perfectly 
as possible, the trace of vapour left behind being exceedingly 
minute. Dry air was then admitted to cleanse the tube. On 
again exhausting, after the first few strokes of the pump a jar 
was felt and a kind of explosion heard, while dense volumes of 
blue smoke immediately issued from the cylinders. The action 


BY GASES AXD VAPOUKS. 


29 


was confined to these, and never propagated backwards into 
the experimental tube. 

It is only with bisulphide of carbon that this effect has been 
observed. It may, I think, be explained in the following 
manner:—To open the valve of the piston, the gas beneath it 
must have a certain pressure, which, when suddenly produced, 
is sufficient to cause the combination of the constituents of the 
bisulphide of carbon with the oxygen of the air. Such a com¬ 
bination certainly takes place, for the odour of sulphurous acid 
is unmistakable amid the fumes. 

To test this idea I tried the effect of compression in the air- 
syringe. A bit of tow or cotton wool moistened with bisulphide 
of carbon, and placed in the syringe, emitted a bright flash 
when the air was compressed. By blowing out the fumes with 
a glass tube, this experiment may be repeated twenty times 
with the same bit of cotton. 

It is not necessary even to let the moistened pellet remain 
in the syringe. If the bit of tow or cotton be thrown into the 
syringe, and out again as quickly as it can be ejected, on com¬ 
pressing the air the luminous flash is seen. Pure oxygen pro¬ 
duces a brighter flash than air. These facts are in harmony 
with the above explanation. 

Continuation of Experiments on Vapours. 

Table YII.— Amylene. 

(Unit-measure ^tk of a cubic inch.) 



Absorption per 100 


Absorption per 100 
■ 

Measures 

r - 

Observed 

> 

Calculated 

Measures 

r 

Observed 

Calculated 

1 

3-4 

4-3 

6 

26-5 

25-8 

2 

8-4 

8-6 

7 

30-6 

30*1 

3 

12 

12-9 

8 

, 35 3 

34-4 

4 

16-5 

17-2 

9 

39 

387 

5 

21*6 

21-5 

10 

44 

43 


Por these quantities the absorption is proportional to the 
density, but for large quantities the usual deviation is observed 
as shown by the following observations : — 

By mercurial gauge. 

Pressure Deflection Absorption per 100 

£ inch 60 157 

1 inch 65 216 







30 


THE ABSORPTION AND RADIATION OF HEAT 


Did the proportion hold good, the absorption for an inch of 
pressure ought to be 314, instead of 216. 


Table VIII .—Iodide of Ethyl, 


(Unit-measure —th of a cubic inch.) 


Absorption per 10 0 


Measures 

Observed 

Calculated 

1 

5-4 

51 

2 

10-3 

10-2 

3 

16*8 

15-3 

4 

22*2 

20-4 

6 

26-6 

255 


Absorption per 100 


Measures 

t — 

Observed 

Calculated 

6 

318 

30-6 

7 

35-6 

359 

8 

40 

40-8 

9 

44 

459 

10 

475 

51 


By mercurial gauge. 

\ 

Pressure Deflection Absorption per 100 

£ inch 56-3 94 

1 inch 582 120 


Table IX .—Iodide of Methyl. 

(Unit-measure T Lth of a cubic inch.) 



Absorption per 100 

A* 



Absorption per 100 

Measures 

f 

Observed 

Calculated 


Measures 

t - 

Observed 

- N 

Calculated 

1 

3-5 

3-4 


6 

20-5 

20-4 

2 

7 

68 


7 

24 

23-8 

3 

10-3 

10-2 


8 

26-3 

27-2 

4 

15 

13-6 


9 

30 

30-6 

5 

17'5 

17 


10 

32-3 

34 



By mercurial gauge. 




Pressure 

Deflection 


Absorption per 100 



\ inch 

o 

48o 


60 



1 inch 

56-5 


96 



Table X .—Iodide of Amyl. 

(Unit-measure J-th of a cubic inch.) 



Absorption per 100 

Measures 

r 

Observed 

Calculated 

1 

06 

0-57 

2 

1 

M 

3 

1-4 

1-7 

4 

2 

2-3 

5 

3 

2-9 



Absorption per 100 

Measures 

Observed 

Calculated 

6 

3*8 

3-4 

7 

4-5 

4 

8 

5 

4-6 

9 

5 

5-1 

10 

58 

57 

















BY GASES AKD VAPOURS. 


31 


The deflections here are very small; the substance, however, 
possesses such feeble volatility that the pressure of a measure 
of its vapour, when diffused through the experimental tube, 
must be infinitesimal. With the specimen examined, it was not 
practicable to obtain a pressure sufficient to cause the mercury 
gauge to sink half-an-inch ; hence no observations of this kind 
are recorded. 

Table XI .—Chloride of Amyl. 


(Unit-measure ^th of a cubic inch.) 


• 

Absorption per 100 

A 


Absorption per 100 

Measures 

r 

Observed 

A 

Calculated 

Measures 

r 

Observed 

■> 

Calculated 

1 

1-3 

1-3 

6 

85 

7-8 

2 

3 

2-6 

7 

9 

91 

3 

3-8 

3 9 

8 

10-9 

10-4 

4 

5-1 

5-2 

9 

11-3 

11*7 

5 

6-8 

6-5 

10 

12-3 

13 


By mercurial gauge. 

Deflection Absorption per 100 

59 137 

not practicable. 

Table XII.— Benzol. 


(Unit-measure ^th of a cubic inch.) 



Absorption per 100 

A— 


Absorption per 100 

Measures 

Observed 

A 

Calculated 

Measures 

r 

Observed 

Calculated' 

1 

4o 

4’5 

11 

47 

49 

2 

9-5 

9 

12 

49 

54 

3 

14 

13*5 

13 

51 

58-5 

4 

18-5 

18 

14 

54 

63 „ 

5 

22*5 

22*5 

15 

56 

67*5 

6 

27-5 

27 

16 

59 

72 

7 

31-6 

31*5 

17 

63 

76-5 

8 

35-5 

36 

18 

67 

81 

9 

39 

40 

19 

69 

85-5 

10 

44 

45 

20 

72 

90 


Up to the 10th measure, or thereabouts, the proportion 
between density and absorption holds good, from which onwards 
the deviation from the law gradually augments. 


By mercurial gauge. 

Pressure Deflection Absorption per 100 

54 78 

57 103 


Pressure 

£ inch 
1 inch 


£ inch 
1 inch 










Measures 

1 

2 

3 

4 

5 

Measure; 

1 

2 

3 

4 

5 

Measun 

1 

2 

3 

4 

5 


THE ABSORPTION AND RADIATION OF HEAT 


Table XIII. — Methylic Alcohol. 


(Unit-measure ~th of a cubic inch.) 


Absorption per 100 


Absorption per 100 
--- A ---\ 

f - ' 

Observed Calculated 

Measures 

r 

Observed 

Calculated 

10 

10 

6 

63-5 

60 

20 

20 

7 

69-2 

70 

30 

30 

8 

71-5 

80 

40*5 

40 

9 

78 

90 

49 

60 

10 

84 

100 


By mercurial gauge. 



Pressure 

Deflection Absorption per 100 


^ inch 

o 

68-8 

133 


1 inch 

60-5 

168 



Table XIY 

• 

-Formic Ether. 



(Unit-measure 

ith of 

a cubic inch.) 



Absorption per 100 




Absorption 

X 

per 100 

( 

Observed 

-\ 

Calculated 



Measures 

f 

Obsei'ved 

Calculated 

8 

i o 



6 

39-5 

45 

16 

15 



7 

45 

52-5 

22-5 

22-5 



8 

48 

60 

30 

30 



9 

50-2 

67-5 

35*2 

37-5 



10 

53-5 

75 


By mercurial gauge. 



Pressure 


Deflection Absorption per 100 


£ inch 


58-8 

133 


1 inch 


62*5 

193 



Table XV.- 

(Unit-measure 
Absorption per 100 

H h 

** ■? 
- o o 

pionate of Ethyl. 

f a cubic inch.) 

Absorption per 100 

—A_ 

Observed 

■\ 

Calculated 

Measures 

Observed 

Calculated 

7 

7 


6 

38-8 

42 

14 

14 


7 

41 

49 

21-8 

21 


8 

42*5 

56 

28-8 

28 


9 

44*8 

63 

34*4 

35 


10 

46*5 

70 


By mercurial gauge. 

Pressure Deflection Absorption per 100 

£ inch 60*5 168 

1 inch not practicable. 

















33 


BY GASES AND VAPOURS. 


Table XY1.— Chloroform. 

(Unit-measure ^th of a cubic inch.) 



Absorption per 100 

-- -A_ 


Absorption per 100 

Measures 

Observed 

Calculated 

Measures 

t - 

Observed 

Calculated"' 

1 

45 

4-5 

6 

27 

27 

2 

9 

9 

7 

31-2 

31-5 

3 

13 8 

13-5 

8 

35 

36 

4 

18-2 

18 

9 

39 

40*5 

5 

223 

225 

10 

40 

45 


Subsequent observations lead me to believe that the absorp¬ 
tion by chloroform is a little higher than that given in the 
above table. 


Table XYII.— Alcohol. 

(Unit-measure ^ a cubic inch.) 



Absorption per 100 

,--A__ 


Absorption per 100 

Measures 

t 

Observed 

Calculated' 

Measures 

c 

Observed 

Calculated 

1 

4 

4 

9 

37-5 

36 

2 

7-2 

8 

10 

41-5 

40 

3 

10-5 

12 

11 

45-8 

44 

4 

14 

16 

12 

48 

48 

5 

19 

20 

13 

50-4 

52 

6 

23 

24 

14 

53-5 

56 

7 

285 

28 

15 

55-8 

60 

8 

32 

32 





By mercurial gauge. 

Pressure Deflection Absorption per 100 

£ inch 60 175 

1 inch not practicable. 

The difference between the measurements when equal pressures 
and when equal volumes at the maximum density are made use 
of is here strikingly exhibited. In the case of alcohol, for 
example, a unit-measure of half a cubic inch was needed to obtain 
an effect about equal to that produced by benzol with a 
measure only T V- of a cubic inch in capacity; and yet for a 
common pressure of 0*5 of an inch, alcohol cuts off precisely 
twice as much heat as benzol. There is also an enormous 
difference between alcohol and sulphuric ether when equal 
measures at the maximum density are compared; but to bring 
the alcohol and ether vapours up to a common pressure, the 
density of the former must be many times augmented. Hence 
it follows that when egual pressures of these two substances are 
3 











34 


THE ABSORPTION AND RADIATION OF HEAT 


compared, the difference between them diminishes considerably. 
Similar observations apply to many of the substances whose 
deportment is recorded in the foregoing 1 tables; to the iodide 
and chloride of amyl, for example, and to the propionate of 
ethyl. Indeed it is not unlikely that with equal pressures the 
vapour of a perfectly pure specimen of the substance last men¬ 
tioned would be found to possess a higher absorptive power 
than that of sulphuric ether itself. 


§ 9 . 

Action of Chlorine.—Possible Influence of Vapours on the Interior 

Surface of the Experimental Tube. 

It has been already stated that the experimental tube em - 
ployed in these experiments was of brass, polished within for 
the purpose of augmenting by reflexion the calorific flux, and 
thus bringing into clearer light the action of the feebler 
gases and vapours. Wishing, however, to try the effect of 
chlorine, I admitted a quantity of the gas into the polished 
tube. The needle was deflected with prompt energy; but on 
pumping out,* it refused to return to zero. To cleanse the 
tube, dry air was introduced into it ten times in succession ; 
but the needle pointed persistently to the 40th degree from zero. 
The cause was easily surmised: the chlorine had attacked 
the metal and partially destroyed its reflecting power; the 
absorption by the sides of the tube itself cutting off an amount 
of heat competent to produce the observed deflection. For 
subsequent experiments the interior of the tube had to be 
repolished. 

Though no vapour previously examined had produced a 
permanent effect of this kind, it was necessary to be perfectly 
satisfied that this source of error had not vitiated the other 
experiments. To check the results, therefore, a length of 2 feet 
of the experimental tube was coated carefully on the inside 
with lampblack, and with it were determined the absorptions 
of all the vapours previously examined, at a common pressure of 
0*3 of an inch. A general corroboration was all that was 

* Dense dark fumes rose from the cylinders on this occasion. A similar effect was 
produced by sulphuretted hydrogen. 


BY GASES AND VArOUItS. 


aimed at, and I am satisfied that tlie slight discrepancies 
which the measurements exhibit would disappear, or be ac¬ 
counted for, in a more careful examination. 

In the following table the results obtained with the blackened 
and with the bright tubes are placed side by side, the pressure 

in the former being three-tenths, and in the latter five-tenths 
of an inch. 

Table XVIII. 


Absorption per 100 


Vapour 

t - 

Blackened Tube 

Bright Tube 

Absorption in 
Blackened Tube 

Bisulphide of carbon . 

0’3 pressure 

0*5 pressure 

proportional to 

5 

21 

23 

Iodide of methyl. 

15-8 

60 

71 

Benzol .... 

. 17*5 

78 

79 

Chloroform.... 

17-5 

89 

79 

Iodide of ethyl . 

21-5 

94 

97 

Wood-spirit.... 

265 

123 

120 

Methylic alcohol . 

29 

133 

131 

Chloride of amyl 

80 

137 

135 

Amylene .... 

. 31-8 

157 

143 


The order of absorption is here shown to be the same in both 
tubes, the quantity absorbed in the bright tube being, in 
general, about 4J times that absorbed in the black one. In the 
third column, indeed, I have placed the numbers contained in 
the first column multiplied by 4*5. These results completely dis¬ 
sipate the suspicion that the effects observed with the polished 
tube could be due to a change of the reflecting power of its 
inner surface by the contact of the vapours. 

With the blackened tube the order of absorption of the 
following substances, commencing with the lowest, stood thus:— 

Alcohol, 

Sulphuric ether, 

Formic ether, 

Propionate of ethyl, 

whereas with the bright tube they stood thus:— 

Formic ether, 

Alcohol, 

Propionate of ethyl, • 

Sulphuric ether. 

As already stated, these differences would in all probability 
disappear, or be accounted for on re-examination. Indeed very 








36 THE ABSORPTION AND RADIATION OF nEAT 

slight differences in the purity of the specimens used would he 
more than sufficient to produce the observed differences of 
absorption.* 


§ 10. 

Action of Permanent Gases on Radiant Heat . 

The deportment of oxygen, nitrogen, hydrogen, atmospheric 
air, and olefiant gas has been already recorded. Besides these 
I have examined carbonic oxide, carbonic acid, sulphuretted 
hydrogen, and nitrous oxide. The action of these gases is so 
much feebler than that of any of the vapours referred to in the 
last section, that, in testing the relationship of absorption to 
density, the unit-measures used with the vapours were aban¬ 
doned, the quantities of gas admitted being determined by the 
depression of the mercurial gauge. 


Table XIX .—Carbonic Oxide. 


Pressure in inches 

Observed 

Calculated 

05 

25 

25 

1 

56 

5 

lo 

8 

7 5 

2 

10 

10 

25 

12 

12-5 

3 

15 

15 

3 5 

17 5 

17-5 


Up to a pressure of 3 J inches the absorption by carbonic oxide 
is proportional to the density of the gas. But this proportion 
does not obtain with large quantities of the gas, as shown by 


the following table :— 

# 


Pressure in inches 

Deflection 

Absorption per 100 

5 

o 

18 

18 

10 

32-5 

32-5 

15 

41 

45 


* In illustration of this I may state, that of two specimens of methylic alcohol 
given me by two of riiy chemical friends, the one gave an absorption of 84 and the 
other of 203. The one had been purified with great care, but the other was not pure. 
Both specimens, however, went under the common name of methylic alcohol. I have 
had a special apparatus constructed with a view to examine the influence of ozone on 
the interior of the experimental tube. 



BY GASES AND VAPOURS. 


37 


Table XX.— Carbonic Acid. 


Pressure in inches 

Observed 

Calculated 

05 

5 

3-6 

1 

7o 

7 

1-5 

10-5 

10-5 

2 

14 

14 

2*5 

17-8 

17-5 

3 

21-8 

21 

3-5 

24-5 

24-5 


Here we have the proportion exhibited, but not so with 
larger quantities. 


Pressure in inches 

Deflection 

o 

Absorption 

5 

25 

25 

10 

36 

36 

15 

42*5 

48 


Table XXI .—Sulphuretted Hydrogen. 


Pressure in 
inches 

Absorption per 100 

-A_ 

Pressure in 
inches 

Absorption per 100 

Observed 

A 

Calculated 

Observed 

> 

Calculated 

0 5 

78 

6 

3 

34*5 

36 

1 

12-5 

12 

35 

36 

42 

1*5 

18 

18 

4 

36-5 

48 

2 

24 

24 

4*5 

38 

54 

25 

30 

30 

5 

40 

60 


The proportion here holds good up to a pressure of 2*5 inches, 
when the deviation from it commences and gradually augments. 

Though these measurements were made with all possible care, 
I should like to repeat them. Dense fumes issued from the 
cylinders of the air-pump on exhausting the tube of this gas, 
and I am not at present able to state with confidence that a 
trace of such in a very diffuse form within the tube did not in¬ 
terfere with the purity of the results. 


Table XXII .—Nitrous Oxide. 


Pressure in 
inches 

Absorption per 100 

A- 

Pressure in 
inches 

Absorption per 100 

Observed 

A 

Calculated 

Observed 

Calculated 

0 5 

14-5 

14-5 

3 

45 

87 

1 

23*5 

29 

3-5 

477 

101*5 

1*5 

30 

43-5 

4 

49 

116 

2 

35*5 

58 

4-5 

51-5 

3 30*5 

2-5 

41 

715 

5 

54 

145 


Here the divergence from proportionality is manifest from 
the commencement. 










38 


THE ABSORPTION AND RADIATION OF HEAT 


Remarks on the Experiments of Dr. Franz. 

I promised at the beginning of this memoir to allude to the 
results of Dr. Franz : tliis shall now be done. With, a tube 3 feet 
long and blackened within, an absorption of 3*54 per cent, by 
atmospheric air was observed in his experiments. In my expe¬ 
riments, however, with a tube 4 feet long and polished within, 
which makes the distance traversed by the reflected rays far 
more than 4 feet, the absorption is only one-ninth of the above 
amount. In the experiments of Dr. Franz, moreover, carbonic 
acid appears as a feebler absorber than oxygen. According to 
my experiments, for small quantities the absorptive power of 
carbonic acid is about 150 times that of oxygen; and at the 
atmospheric pressure, carbonic acid probably absorbs nearly 
100 times as much as oxygen. 

The differences between Dr. Franz and me admit of the follow¬ 
ing simple explanation. His source of heat was an argand 
lamp, and the ends of his experimental tube were stopped with 
plates of glass. Now Melloni has shown that fully 61 per cent, 
of the heat rays emanating from a Locatelli lamp are absorbed 
by a plate of glass one-tenth of an inch in thickness. Hence 
the greater portion of the rays issuing from the lamp of 
Dr. Franz was expended in heating the two glass ends 
of his experimental tube. These ends became secondary 
sources of heat which radiated against his pile. On admitting 
cool air into the tube, the partial withdrawal by conduction and 
convection of the heat of the glass plates produced an effect 
exactly the same as that of true absorption. By allowing the 
air in my experimental tube to come into contact with the radia¬ 
ting plate, I have often obtained a deflection of twenty or thirty 
degrees,—the effect being due to the cooling of the plate, and 
not to absorption. It is also certain that, had I, like Dr. Franz, 
used heat from a luminous source, my small absorption of 0*4 
per cent. Tvould have been considerably diminished. 

§ 11 . 

Action of Aqueous Vapour.—Possible Effect of an Atmospheric 
Envelope on the Temperature of a Planet. 

I have now to refer briefly to a point of considerable interest 
—the effect, namely, of our atmosphere on solar and terrestrial 


BY GASES AND VAPOURS. 


39 


heat. In examining the separate effects of the air, carbonic 
aci(J, and aqueous vapour of the atmosphere, on the 20th of 
last November, the following results were obtained:— 

Air sent through the caustic-potash tube and through the 
drying-tubes £>roduced an absorption of about . . 1 

Air direct from the laboratory, containing therefore its 
carbonic acid and aqueous vapour, produced an ab¬ 
sorption of .15 

Deducting the effect of the gaseous acid, it was found that 
the quantity of aqueous vapour diffused through the atmosphere 
on the day in question, produced an absorption at least equal 
to thirteen times that of the atmosphere itself. 

It is my intention to repeat and extend these experiments on 
a future occasion*; but even at present conclusions of great 
importance may be drawn from them. It is exceedingly pro¬ 
bable that the absorption of the solar rays by the atmosphere, as 
established by M. Pouillet, is mainly due to the watery vapour 
contained in the air. The vast difference between the tempera¬ 
ture of the sun at midday and in the evening, is also probably 
due in the main to that comparatively shallow stratum of 
aqueous vapour which lies close to the earth. At noon the 
depth of it pierced by the sunbeams is very small; in the even¬ 
ing very great in comparison. 

The intense heat of the sun’s direct rays on high mountains 
is not, I believe, due to his beams having to penetrate only a 
small depth of air, but to the comparative absence of aqueous 
vapour at those great elevations.f 

But this vapour, which exercises such a destructive action 
on the obscure rays is comparatively transparent to the rays 
of light. Hence the • differential action of the heat coming 
from the sun to the earth, and that radiated from the earth 
into space, is vastly augmented by the aqueous vapour of the 
atmosphere. 

De Saussure, Fourier, M. Pouillet, and Mr. Hopkins regard 
this interception of the terrestrial rays as exercising a most 

* The peculiarities of the locality in which this experiment was made render its 
repetition under other circumstances necessary. 

t As proved by the observations of Welsh and Hooker, reasoned out by Strachey. 

[ 1872 .] 


40 THE ABSORPTION AND RADIATION OF HEAT 

important influence on climate. Now if, as tlie above experi¬ 
ments indicate, the chief influence be exercised by the aqueous 
vapour, every variation of this constituent must produce a 
change of climate. Similar remarks would apply to the 
carbonic acid diffused through the air, while an almost inappre¬ 
ciable admixture of any of the stronger hydrocarbon vapouis 
would powerfully hold back the terrestrial rays and produce cor¬ 
responding climatic changes. It is not, therefore, necessary to 
assume alterations in the density and height of the atmosphere 
to account for different amounts of heat being preserved to the 
earth at different times ; a slight change in the variable consti¬ 
tuents of the atmosphere w r ould suffice. Such changes in fact 
may have produced all the mutations of climate which the 
researches of geologists reveal. However this may be, the 
facts above cited remain firm ; they constitute true causes, the 
extent alone of the operation remaining doubtful.* 

The measurements recorded in the foregoing pages constitute 
only a small fraction of those actually made ; but they fulfil the 
object of the present portion of the inquiry. They establish the 
existence of enormous differences among colourless gases and 
vapours as to their action upon radiant heat; and they also 
show that when the quantities are sufficiently small, the absorp¬ 
tion in the case of each particular vapour is exactly proportional 
to the density. 

The experiments, moreover, furnish us with purer cases of 
molecular action than have been hitherto attained in researches of 
this nature. In both solids and liquids the cohesion of the par¬ 
ticles is implicated; they mutually control and limit each other. 
A certain action, over and above that which belongs to them 
separately, comes into play and embarrasses our conceptions. But 
in the cases above recorded the molecules are perfectly free, and 
we fix upon them individually the effects which the experiments 
exhibit; thus the mind’s eye is directed more firmly than ever 
on those distinctive physical qualities whereby a ray of heat is 
stopped by one molecule and unimpeded by another. 


* On this point, see Section 23, Memoir II. 


BY GASES AND VAPOURS. 


41 


§ 12 . 

RADIATION OF HEAT BY GASES. 

Reciprocal Experiments on Radiation and Absorption. 

It is known that tlie quantity of liglit emitted by a flame 
depends chiefly on the incandescence of solid matter; the bright¬ 
ness of an ignited jet of ordinary gas, for example, being chiefly 
due to the solid particles of carbon liberated in the flame.* 

Melloni found the radiation of heat from his alcohol lamp to 
be greatly augmented by plunging a spiral of platinum wire into 
the flame. He also proved that a bundle of wire placed in the 
current of hot air ascending from an argand chimney gave a 
copious radiation, while when the wire was withdrawn no trace 
of radiant heat could be detected by his apparatus. He con¬ 
cluded from this experiment that air possesses the power of 
radiation in so feeble a degree that our best thermoscopic in¬ 
struments fail to detect this power.f These are the only ex¬ 
periments hitherto published upon this subject, and they are 
negative. 

I have now to record some affirmative ones. The pile, furnished 
with its conical reflector, was placed upon a stand, with a screen 
of polished tin in front of it. An alcohol lamp was placed behind 
the screen, so that its flame was entirely hidden by the latter; on 
rising above the screen, the gaseous column radiated its heat 
against the pile and produced a considerable deflection. The 
same effect was produced when a candle or an ordinary jet of 
gas was substituted for the alcohol lamp. 

The heated products of combustion acted on the pile in the 
above experiments, but the radiation from ordinary undried air 
was easily demonstrated by placing a heated iron spatula or metal 
sphere behind the screen. A deflection was thus obtained 
which, when the spatula was raised to a red heat, amounted to 
more than sixty degrees. No radiation from the spatula to the 
pile, was here possible, and no portion of the heated air itself 
approached the pile, so as to communicate its warmth by contact 
to the latter. 

* By a suitable arrangement the particles may be liberated red-hot , instead of white-, 
hot, each individual particle describing a line of red light. [1872.] 

t La Thermochrose, p. 94. Eurther: ‘No fact is yet known which directly proves 
the emissive power of pure and transparent elastic fluids.’—Taylor’s Scientific Memoirs , 

vol. v. p. 6ol. 


42 


THE ABSORPTION AND RADIATION OF HEAT 


My next care was to examine whether different gases possessed 
different powers of radiation ; and for this purpose the following 
arrangement was devised. P (fig. 1) represents the thermo¬ 
electric pile with its two conical reflectors; S is a double screen 
of polished tin; A is an argand burner consisting of two con¬ 
centric rings perforated with orifices for the escape of the gas; 
C is a heated copper hall; the tube 11 leads to a gas-holder 
containing the gas to be examined. When the ball C is placed 
on the argand burner, it of course heats the air in contact 
with it; an ascending current is established, which acts on the 


Fig. l. 



pile as in the experiments last described. It was found 
necessary to neutralize this radiation, and for this purpose a 
large Leslie’s cube L, filled with water a few degrees above 
the temperature of the air, was allowed to act on the opposite 
face of the pile. 

When the needle was thus brought to zero, the cock of the 
gas-holder was turned on; the gas passed through the burner, 
came into contact with the ball, and ascended afterwards in a 
heated column in front of the pile. The galvanometer was now 
observed, and the limit of the arc through which its needle was 





























































BY GASES AND VAPOURS. 


43 


jirged was noted. It is needless to remark that the ball was 
entirely hidden by the screen from the thermo-electric pile, 
and that, even were this not the case, the mode of compensa¬ 
tion adopted would still give us the pure action of the gas. 

The results of the experiments are given in the following 
table, the figure appended to the name of each gas marking 
the number of degrees through which the radiation from the 
latter urged the needle of the galvanometer*:— 


Degrees 


Air. 




. 0 

Oxygen . 




. 0 

Nitrogen ..... 




. 0 

Hydrogen ..... 




0 

Carbonic oxide .... 




. 12 

Carbonic acid .... 




. 18 

Nitrous oxide ..... 




. 29 

Olefiant gas ..... 




. 53 


The radiation from air, it will be remembered, was neutralized 
by the large Leslie’s cube, and hence the 0° attached to it 
merely denotes that the propulsion of air from the gas-holder 
through the argand burner did not augment the effect. Oxygen, 
hydrogen, and nitrogen, sent in a similar manner over the ball, 
were equally ineffective. The other gases, however, not only 
exhibit a marked action, but also marked differences of action. 
Their radiative powers follow precisely the same order as their 
powers of absorption. In fact, the deflections actually pro¬ 
duced by their respective absorptions at a common pressure of 
5 inches are as follows 


Air 

Oxygen . 
Nitrogen 
Hydrogen 
Carbonic oxide 
Carbonic acid 
Nitrous oxide 
Olefiant gas . 


A fraction of a degree 




n 


. 18° 
. 25° 
. 44° 
. 61° 




It would be easy to give these experiments a more elegant 
form, and to arrive at greater accuracy, which I intend to flo on 
a future occasion; but my object now is simply to establish the 
general order of the radiative powers of these gases, as con¬ 
trasted with their powers of absorption. 

* I have also rendered these experiments on radiation visible to a large assembly 
They may be readily introduced in lectures on radiant heat. 

















44 


THE ABSORPTION AND RADIATION OP HEAT 


§13. 

The Varnishing of Polished Metal Surfaces by Gases. 

When the polished metallic face of a Leslie’s cube is turned 
towards a thermo-electric pile, the effect produced is incon¬ 
siderable, but it is greatly augmented when a coat of var¬ 
nish is laid upon the polished surface. Now a film of gas 
may be employed instead of the coat of varnish. Such a 
cube, containing boiling water, had a polished silver face 
turned towards the pile, and its effect on the galvanometer 
neutralized in the usual manner. The needle being at 0°, a 
film of olefiant gas, issuing from a narrow slit, was caused to 
pass over the metal. The consequent radiation produced a 
deflection of 45°. When the gas was cut off, the needle returned 
accurately to 0°. 

Beciprocally, absorption by a coating film of gas may be 
shown by filling a cube with cold water, but not so cold as to 
produce the precipitation of the aqueous vapour of the air, and 
allowing the pile to radiate against it. A gilt copper ball, 
cooled in a freezing mixture, being placed in front of the pile, 
its effect was neutralized by presenting a beaker containing a 
little iced water to the face opposite. A film of olefiant gas 
was sent over the ball, but the consequent deflection proved 
that the absorption by the ball, instead of being greater, was 
less than before. On examination, the ball was found coated 
with, a crust of ice, which is one of the best absorbers of radiant 
heat. The olefiant gas, being warmer than the ice, partially 
neutralized its action. When, however, the temperature of 
the ball was only a few degrees lower than that of the atmo¬ 
sphere, and its surface quite dry, the film of gas was found, 
like a film of varnish, to augment the absorption. 


§ 14. 

First Observation of the Radiation of a Vapour heated 

dynamically. 


A remarkable effect, which contributed at first to the Com¬ 
plexity of the experiments, can now be explained. Conceive the 
experimental tube exhausted and the needle at zero; conceive a 


BY GASES AjND VAPOURS. 


45 


small quantity of alcohol or ether vapour admitted ; it cuts off 
a portion of the heat from one source, and the o}3posite source 
triumphs. Let the consequent deflection be 45°. If dry air be 
now admitted till the tube is filled, its normal effect of course 
would be slightly to augment the absorption and make the 
deflection greater. But the following action is really ob¬ 
served :—When the air first enters, the needle, instead of 
ascending, descends ; it falls to 26°, as if a portion of the heat 
originally cut off had been restored. At 26°, however, the. 
needle stops, turns, moves quickly upwards, and takes up a 
permanent position a little higher than 45°. Let the tube now 
be exhausted, the withdrawal of the mixed air and vapour ought 
of course to restore the equilibrium with which we started; but 
the following effects are observed:—When the exhaustion com- 
mences, the needle moves upwards from 45° to 54°; it then 
halts, turns, and descends speedily to 0°, where it permanently 
remains. 

After many attempts to account for the anomaly, I proceeded 
thus :—A thermo-electric couple was soldered to the external 
surface of the experimental tube, and its ends connected with a 
galvanometer. When air was admitted, a deflection was pro¬ 
duced, showing that the air, on entering the vacuum, was 
heated. On exhausting, the needle was also deflected, showing 
that the interior of the tube was chilled. These are indeed 
known effects; but I was anxious to make myself perfectly sure 
of them. I subsequently had the tube perforated and delicate 
thermometers screwed into it airtight. On filling the tube 
the thermometric columns rose, on exhausting it they sank, the 
range between the maximum and minimum amounting in the 
case of air to 5° Fahr. 

Hence the following explanation of the above singular effects. 
The absorptive power of the ether or alcohol vapour is very 
great, and its radiative power is equally so. The heat generated 
by the air on its entrance is communicated to the vapour, which 
thus becomes a temporary source of radiant heat, and diminishes 
the deflection produced in the first instance by its presence. 
The reverse occurs when the tube is exhausted; the vapour is 
then chilled, its great absorptive action on the heat radiated 
from the adjacent face of the pile comes into play, the original 
absorption being apparently augmented. In both cases, how- 


46 


THE ABSORPTION AND RADIATION OF HEAT 


ever, the action is transient; the vapour soon loses the heart 
communicated to it, and soon gains the heat which it has lost, 
and matters then take their normal course.* 

§ 15 . 

On the Physical Connexion of Radiation , Absorption, and 

Conduction. 

Notwithstanding the great accessions of late years to our 
knowledge of the nature of heat, we are as yet, I believe, quite 
gnorant of the atomic conditions on which radiation, absorp¬ 
tion, and conduction depend. What are the specific qualities 
which cause one body to radiate copiously and another feebly ? 
Why, on theoretic grounds, must the equivalence of radiation 
and absorption exist? Why should a highly diathermanous 
body, as shown by Mr. Balfour Stewart, be a bad radiator, and 
an athermanous body a good radiator? How is heat con¬ 
ducted? and what is the strict physical meaning of good 
conduction and bad conduction ? Why should good conductors 
be, in general, bad radiators, and bad conductors good radiators ? 
These, and other questions, referring to facts more or less 
established, have still to receive their complete answers. It was 
less with a hope of furnishing such than of shadowing forth 
the possibility of uniting these various effects by a common 
bond, that I submitted the following reflexions to the notice of 
the Royal Society. 

In the experiments recorded in the foregoing pages, we have 
dealt with free atoms, both simple and compound, and it has 
been found that in all cases absorption takes place. The 
meaning of this, according to the dynamical theory of heat, is 
that no atom is capable of existing in vibrating aether without 
accepting a portion of its motion. We may, if we wish, 
imagine a certain roughness of the surface of the atoms which 
enables the aether to bite them and carry the atom along with 
it. But no matter what the quality may be which enables any 
atom to accept motion from the agitated aether, the same quality 
must enable it to impart motion to still aether when it is plunged 
in the latter and agitated. It is only necessary to imagine a 
body immersed in water to see that this must be the case. 

* Under the head Dynamic Radiation and Absorption, this result is fully developed 
in subsequent memoirs. 


47 


BY GASES AND VAPOURS. 


There is a polarity here as rigid as that of magnetism. 
From the existence of absorption, we may on theoretic grounds 
infallibly infer a capacity for radiation; from the existence of 
radiation, we may with equal certainty infer a capacity for 
absorption; and each of them must be regarded as the measure 
of the other.* 

This reasoning, founded simply on the mechanical relations 
of the aether and the atoms immersed in it, is completely verified 
by experiment. Great differences have been shown to exist 
among gases as to their powers of absorption, and precisely 
similar differences as regards their powers of radiation. But 
what specific property is it which makes one free molecule a 
strong absorber, while another offers scarcely any impediment 
to the passage of radiant heat ? I think the experiments throw 
some light upon this question. If we inspect the results above 
recorded, we shall find that the elementary gases—hydrogen, 
oxygen, nitrogen—and the mixture atmospheric air, possess 
absorptive and radiative powers beyond comparison less than 
those of the compound gases. Uniting the atomic theory with 
the conception of an aether, this result appears to be exactly 
what ought to be expected. Taking Dalton’s idea of an elemen¬ 
tary body as a single sphere, and supposing such a sphere to be 
set in motion in still aether, or placed without motion in moving 
aether, the communication of motion by the atom in the first 
instance, and the acceptance of it in the second, must be less 
than when a number of such atoms are grouped together and 
move as a system. Thus we see that hydrogen and nitrogen, 
which, when mixed together, produce a small effect, when chemi¬ 
cally united to form ammonia, produce an enormous effect. 
Thus oxygen and hydrogen, which, when mixed in their elec¬ 
trolytic proportions, show a scarcely sensible action, when 
chemically combined to form aqueous vapour exert a powerful 
action. So also with oxygen and nitrogen, which, when mixed, 
as in our atmosphere, both absorb and radiate feebly, when 
united to form oscillating systems, as in nitrous oxide, have 
their powers vastly augmented. Pure atmospheric air, of 5 
inches’ mercury pressure, does not effect an absorption equivalent 

* This was written long before Kirchhoff’s admirable papers on the relation of 
emission to absorption were known to me. The vibrating period is not here taken 
into account, but simply the amount of molecular motion capable of being received 
or imparted. 


48 THE ABSORPTION AND RADIATION OF HEAT 

to more than one-fifth of a degree, while nitrons oxide of the 
same pressure effects an absorption equivalent to fifty-one 
degrees. Hence the absorption by nitrous oxide at this pres¬ 
sure is about 250 times that of air. 

Ho fact in chemistry carries the same conviction to my mind, 
that air is a mixture and not a compound , as that just cited. 

In like manner, the absorption by carbonic oxide of 5 inches 
pressure is nearly 100 times that of oxygen alone ; the absoiption 
by carbonic acid is about 150 times that of oxygen; while the 
absorption by olefiant gas of this pressure is 1,000 times that 
of its constituent hydrogen. Even the enormous action last 
mentioned is surpassed by the vapours of many of the volatile 
liquids, in which the atomic groups are known to attain their 
highest degree of complexity. 

But, besides molecular complexity, another important con¬ 
sideration remains. All the gases and vapours hitherto men¬ 
tioned are transparent to light; that is to say, the waves of the 
visible spectrum pass among them without sensible absorption. 
Hence it is plain that their absorptive power depends on the 
periodicity of the undulations which strike them. At this 
point the present inquiry connects itself with the experiments of 
Niepce,* the observations of Foucault,f the theoretic notions of 
Euler, Angstrom,J Stokes, and Thomson, and those splendid 
researches of Kirchhoff and Bunsen which so immeasurably 
extend our experimental range. By Kirchlioff it has been con¬ 
clusively shown that every atom absorbs in a special degree 
those waves which are synchronous with its own periods of 
vibration. Now, besides presenting broader sides to the aether, 
the association of simple atoms to form groups must, as 
a general rule, render their motions through the aether more 
sluggish, and tend to bring the periods of oscillation into 
isochronism with the slow^ undulations of obscuie heat, thus 
enabling the molecules to absorb more effectually such ia^s as 
have been made use of in our experiments. 

Let me here state briefly the grounds which induce me to 
conclude that an agreement in period alone is not sufficient to 
cause powerful absorption and radiation that in addition to 

* Referred to by Angstrom. See below. 

f Annales de Chimie, 1860, vol. lviii. p. 476. 

^ Poggendorff’s Atmodcv.^ vol. xciv. p. 41. J?Jiilosophiccil ^Idycizinc, vol. ix. p, 327. 


BY GASES AND VAPOURS. 


49 


this the molecules must he so constituted as to furnish points 
cCappui to the aether. The heat of contact is accepted with 
extreme freedom by rock-salt, but a plate of the substance once 
heated requires a great length of time to cool. This surprised 
me when I first noticed it. But the effect is explained by the 
experiments of Mr. Balfour Stewart, by whom it has been 
proved that the radiative power of heated rock-salt is extremely 
feeble. Periodicity can have no influence here, for the aether is 
capable of accepting and transmitting impulses of all periods; 
and the fact that rock-salt requires more time to cool than alum 
simply proves that the molecules of the former glide through 
the aether with comparatively small resistance, and thus continue 
moving for a long time; while those of the latter speedily 
communicate to it the motion which we call radiant heat. 
This power of gliding through still aether possessed by the 
rock-salt molecules, must of course enable the moving aether to 
glide round them, and no coincidence of period could, I think, 
make such a body a powerful absorber.* 

Many chemists, I believe, are disposed to reject the idea of 
an atom, and to adhere to that of equivalent proportions merely. 
They figure the act of combination as a kind of interpenetration 
of one substance by another. But this is a mere masking of 
the fundamental phenomenon. The value of the atomic theory 
consists in its furnishing the physical explanation of the law of 
equivalents : assuming the one, the other follows; f and assum¬ 
ing the act of chemical union as Dalton figured it, we see that 
it blends harmoniously with the perfectly independent concep¬ 
tion of an aether, and enables us to reduce the phenomena 
of radiation and absorption to the simplest mechanical prin¬ 
ciples. 

Considerations similar to the above may, I think, be applied 
to the phenomena of conduction. In the 4 Philosophical Maga¬ 
zine 5 for August 1853, I have described an instrument used 
in examining the transmission of heat through cubes of wood 
and other substances. When engaged with this instrument, I 


* With reference to the question of vibrating period and molecular form, see 
Fragments of Science , earlier editions, p. 210 et seq., or Radiation, p. 52 et seq. 

f For the further treatment of this subject, see Fragments of Science, earlier 
editions, pp. 135 and 136. See also Faraday as a Discoverer, cheap edition, p. 146 
et seq. 


4 


50 


THE ABSORPTION AND RADIATION OF IIEAT 


had also cubes of various crystals prepared, and determined 
approximately their powers of conduction. With one exception, 
it was found that the conductivity augmented with the diather¬ 
mancy. The exception was that a cube of very perfect rock- 
crystal conducted heat slightly better than a cube of rock- 
salt. The latter, however, had a very high conductive power; 
in fact rock-salt, calcareous spar, glass, selenite, and alum stood 
in these experiments, and as regards conductivity, exactly in their 
order of diathermancy in the experiments of Melloni. Con¬ 
siderations have been already adduced which show that the mole¬ 
cules of rock-salt glide with facility through the sether; but the 
ease of motion which these molecules enjoy must facilitate 
their mutual collision. Their motion, instead of being ex¬ 
pended on the sether between them, and communicated by it 
to the external sether, is in great part transferred directly from 
particle to particle, or in other words, is freely conducted. 
When a molecule of alum, on the contrary, oscillates, it 
produces a swell in the intervening sether, which swell is in 
part transmitted, not to the molecules, but to the general 
sether of space, and thus lost as regards conduction. This 
lateral waste prevents the motion from penetrating the alum 
to any great extent, and the substance is what we call a bad 
conductor.* 

Such considerations as these could hardly occur without 
carrying the mind to the kindred question of electric conduc¬ 
tion ; but the speculations have been pursued sufficiently far for 
the present, and they must now abide the judgment of those 
competent to decide whether they are the mere emanations of 
fancy, or a fair application of principles which are acknowledged 
to be secure. 

The present paper, I may remark, embraces only the first 
section of these researches. 

* In tlie above considerations regarding conduction, I have limited myself to the 
illustration furnished by two compound bodies ; but the elementary atoms also 
differ among themselves as regards their powers of accepting motion from the 
tether and of communicating motion to it. I should infer, for example, that the 
atoms of platinum encounter more resistance in moving tlirough the sether than the 
atoms of silver; simply because platinum is a worse conductor than silver. 


BY GASES AND VAPOURS. 


51 


Supplementary Remarks, 1872. 

For the use of the younger student here follow three extracts 
from my book oh 4 Heat, a Mode of Motion.’ 

1. Note on the construction of the thermo-electric pile. 

2. Note on the construction of the galvanometer. 

3. Remarks on the different values of galvanometric 

degrees. 

Also Melloni’s account of the mode of calibrating a gal¬ 
vanometer ; in other words, of reducing all its degrees, high 
and low, to a common value, extracted from 4 La Thermo- 
chrose.’ 


Fig. 2. 


1. JSote on the Construction of the Thermo-electric Pile . 

Let ab (fig. 2) be a bar of antimony, and b c a bar of bismuth, 
soldered together at b. Let the free ends a and c be united by a 
wire, adc. On warming the place of junction, b, 
an electric current is generated, the direction of 
which is from bismuth to antimony (or against the 
alphabet), across the junction, and from antimony 
to bismuth (or with the alphabet), through the 
connecting wire, adc. The arrows indicate the 
direction of the current. 

If the junction b be chilled , a current is gene¬ 
rated opposed in direction to the former. The 
figure represents what is called a thermo-electric 
pair or couple. 

By the union of several thermo-electric pairs a 
more powerful current can be generated than that 
obtained from a single pair. Fig. 3 (next page), for example, 
represents such an arrangement, in which the shaded bars are 
supposed to be all of bismuth, and the unshaded ones of anti¬ 
mony. On warming all the junctions, b,b, &e., a current is 
generated in each, and the sum of these currents, all of which 
flow in the same direction, produces a stronger resultant current 
than that obtained from a single pair. 

The V formed by each pair need not be so wide as it is 
shown in fig. 3 ; it may be contracted without prejudice to the 




52 


THE ABSORPTION AND RADIATION OF HEAT 


couple. And if it is desired to pack several pairs into a small 
compass, each separate couple may be arranged as in fig. 4, where 

Fig. 3. 



Fig. 4. 



the black lines represent small bismuth bars, and the white ones 
small bars of antimony. They are soldered together at the ends, 
and throughout their length are usually separated by strips of 

paper merely. A collection of pairs thus 
compactly set together constitutes a thermo- 
, electric pile, a drawing of which is given in 
fig. 5. 

The current produced by heat being always 
from bismuth to antimony across the heated junction, a moment s 
inspection of fig. 3 will show that when any one of the junctions 
A, A, is heated, a current is excited opposed in direction to that 
generated when the heat is applied to the junctions b, b. Hence 
in the case of the thermo-electric pile, the effect of heat falling 

upon its two opposite faces is to produce 
currents in opposite directions. If the 
temperature of the two faces be alike, 
Ej they neutralise each other, no matter 
how highly they may be heated abso¬ 
lutely; but if one of them be warmer 
than the other, a current is produced. 
The current is thus due to a difference of temperature between 
the two faces of the pile, and within certain limits the strength 


Fig. 5. 



of the current is exactly proportional to this difference. 

From the junction of almost any other two metals, thermo¬ 
electric currents may be obtained, but they aie most leadily 
generated by the union of bismuth and antimony.* 


* The discovery of thermo-electricity is due to Thomas Seebeck, Frofessor in the 
University of Berlin. Nobili constructed the first thermo-electric pile; but in 
Melloni’s hands it became an instrument so important as to supersede all others in 
researches on radiant heat. 



































BY GASES JlSJ) VAPOURS. 


53 


2. Note on the Construction of the Galvanometer . 


Fig. 6. 


The existence and direction of an electric current are shown 
by its action upon a freely suspended magnetic needle. 

But such a needle is held in the magnetic meridian by the 
magnetic force of the earth. Hence, to move a single needle, the 
current must overcome the magnetic force of the earth. 

Very feeble currents are incompetent to do this in a sufficiently 
sensible degree. The following two expedients are, therefore, 
combined to render sensible the action of such currents :— 

The wire through which the current flows is coiled so as to 
surround the needle several times ; the needle must swing freely 
within the coil. The action of the single current is thus multi¬ 
plied. 

The second device is to neutralise the directive force of the 
earth, without prejudice to the magnetism of the needle. This 
is accomplished by using two needles 
instead of one, attaching them to a 
common vertical stem, and bringing 
their opposite poles over each other, 
the north end of the one needle and 
the south end of the other being 
thus turned in the same direction. 

The double needle is represented in 

fig. 6. 

It must be so arranged that one of the needles shall be within 
the coil through which the current flows, while the other needle 
swings freely above the coil, the vertical connecting-piece passing 
through an appropriate slit in the coil. Were both the needles 
within the coil, the same current would urge them in opposite 
directions, and thus one needle would neutralise the other. But 
when one is within and the other without, the current urges 
both needles in the same direction. 

The way to prepare such a pair of needles is this. Magnetise 
both of them to saturation ; then suspend them in a vessel, or 
under a shade, to protect them from air-currents. The system 
will probably set in the magnetic meridian, one needle being in 
almost all cases stronger than the other. Weaken the stronger 
needle carefully by the touch of a second smaller magnet. When 


£ 


n 


n 











54 


THE ABSORPTION AND RADIATION OF HEAT 


the needles are precisely equal in strength, they will set at right 
angles to the magnetic meridian. 

It might be supposed that when the needles are equal in 
strength, the directive force of the earth would be completely 
annulled, that the double needle would be perfectly astatic , 
and perfectly neutral as regards direction; obeying simply the 
torsion of its suspending fibre. This would be the case if the 
magnetic axes of both needles could be caused to lie with mathe¬ 
matical accuracy in the same vertical plane. In practice this 

is almost impossible; the axes 
always cross each other. Let n s, 
n' s' (fig. 7) represent the axes of 
two needles thus crossing, the 
magnetic meridian being parallel 
to me; let the pole n be drawn 
by the earth’s attractive force 
in the direction n m ; the pole s' 
being urged by the repulsion of 
the earth in a precisely opposite 
direction. When the poles n and 
s' are of exactly equal strength, 
it is manifest that the force acting 
on the pole s', in the case here 
supposed, would have the advan¬ 
tage as regards leverage, and would therefore overcome the 
force acting on n. The crossed needles would therefore turn 
away still further from the magnetic meridian, and a little 
reflexion will show that they cannot come to rest until the line 
which bisects the angle enclosed by the needles is at right 
angles to the magnetic meridian. 

This indeed is the test of perfect equality as regards the mag¬ 
netism of the needles ; but in bringing them to this state of per¬ 
fection we have often to pass through various stages of obliquity 
to the magnetic meridian. In these cases the superior strength 
of one needle is compensated by an advantage, as regards 
leverage, possessed by the other. By a happy accident a touch 
is sometimes sufficient to make the needles perfectly equal; but 
many hours are often expended in securing this result. It is 
only of course in very delicate experiments that this perfect 
equality is needed; but in such experiments it is essential. 


Fig. 7. 


:vi] 








BY GASES AND VAPOURS. 


55 


3. Remarks on the different Values of Galvanometric Degrees. 

The needle being at zero, let us suppose a quantity of heat 
to fall upon the pile, sufficient to produce a deflection of one 
degree. Suppose the quantity to be afterwards augmented, 
so as to produce deflections of two degrees, three degrees, four 
degrees, five degrees ; then the amounts of heat which produce 
these deflections stand to each other in the ratios of 1 : 2 : 3: 
4:5: the heat which produces a deflection of 5° being ex¬ 
actly five times that which produces a deflection of 1°. But 
this proportionality exists only so long as the deflections 
do not exceed a certain magnitude. For, as the needle is drawn 
more and more aside from zero, the current acts upon it at an 
ever-augmenting disadvantage. The case is illustrated by a 
sailor working a capstan ; he always applies liis strength at right 
angles to the lever, for, if he applied it obliquely, only a portion 
of that strength would be effective in turning the capstan round. 
And in the case of our electric current, when the needle is very 
oblique to the current’s direction, only a portion of its force is 
effective in moving the needle. Thus it happens, that though 
the quantity of heat may be, and, in our case, is, accurately 
expressed by -the strength of the current which it excites, still 
the larger deflections, inasmuch as they do not^’ive us the action 
of the whole current, but only of a part of it, cannot be a true 
measure of the amount of heat falling upon the pile. 

The galvanometer here employed is so constructed that the 
angles of deflection up to 30° or thereabouts, are proportional 
to the quantities of heat; the quantity necessary to move the 
needle from 29° to 30° is sensibly the same as that required to 
move it from 0° to 1°. But beyond 30° the proportionality 
ceases. The quantity of heat required to move the needle from 
40° to 41° is three times that necessary to move it from 0° to 1° ; 
to deflect it from 50° to 51° requires five times the heat necessary 
to move it from 0° to 1°; to deflect it from 60° to 61° requires 
about seven times the heat necessary to move it from 0° to 1°; to 
deflect it from 70° to 71° requires eleven times, while to move it 
from 80° to 81° requires more than fifty times the heat necessary 
to move it from 0° to 1°. Thus, the higher we go, the greater 
is the quantity of heat represented by a degree of deflection; 
the reason being, that the force which then moves the needle is 


56 


THE ABSORPTION AND RADIATION OF HEAT 


only a fraction of the force of the current really circulating in 
the wire, and hence represents only a fraction of the heat falling 
upon the pile. 


4. Calibration of the Galvanometer. 

The following method of calibrating the galvanometer is re¬ 
commended by Melloni as leaving nothing to be desired in regard 
to facility, promptness, and precision. His own statement of 
the method, translated from 4 La Thermochrose, 5 p. 59, is as 
follows: — 

Two small vessels, v v, are half-filled with mercury, and 

connected separately, by two short wires, 
Fig - 8 - with the extremities G G of the galvano¬ 

meter. The vessels and wires thus dis¬ 
posed make no change in the action of 
the instrument; the thermo-electric cur¬ 
rent being freely transmitted, as before, 
from the pile to the galvanometer. But 
if, by means of a wire f, a communica¬ 
tion be established between the two 
vessels, part of the current will pass 
through this wire and return to the 
pile. The quantity of electricity circulating in the galvano¬ 
meter will be thus diminished, and with it the deflection of 
the needle. 

Suppose, then, that by this artifice we have reduced the gal- 
vanometric deviation to its fourth or fifth part; in other words, 
supposing that the needle, being at 10 or 12 degrees, under the 
action of a constant source of heat, placed at a fixed distance 
from the pile, descends to 2 or 3 degrees, when a portion of 
the current is diverted by the external wire; I say, that by 
causing the source to act from various distances, and observing 
in each case the total deflection, and the reduced deflection, we 
have all the data necessary to determine the ratio of the 
deflections of the needle, to the forces which produce these 
deflections. 

To render the exposition clearer, and to furnish, at the same 
time, an example of the mode of operation, I will take the 
numbers relating to the application of the method to one of my 
thermo-multipliers. 




BY GASES AND VAPOURS. 


57 


The external circuit being interrupted, and the source of heat 
being sufficiently distant from the pile to give a deflection not 
exceeding 5 degrees of the galvanometer, let the wire be placed 
from v to v ; the needle falls to 1*5°. The connection between 
the two vessels being again interrupted, let the source be brought 
near enough to obtain successively the deflections:— 

5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°. 

Interposing after each the same wire between v and v, we obtain 
the following numbers :— 

1-0°, 3°, 4*5*°, 6-3°, 8-4°, 11*2°, 15*3°, 22-4°, 29*7°. 

Assuming the force necessary to cause the needle to describe 
each of the first degrees of the galvanometer to be equal to 
unity, we have the number 5 as the expression of the force cor¬ 
responding to the first observation. The other forces are easily 
obtained by the proportions :— 

1*5 : 5 = a : x = T 5 ^ a = 3*333.* 

where a represents the deflection when the exterior circuit is 
closed. We thus obtain 


5, 10, 15*2, 21, 28, 37*3 


for the forces, corresponding to the deflections, 


5°, 10°, 15°, 20°, 25°, 30°. 


In this instrument, therefore, the forces are sensibly propor¬ 
tional to the arcs, up to nearly 15 degrees. Beyond this, the 
proportionality ceases, and the divergence augments as the arcs 
increase in size. 

The forces belonging to the intermediate degrees are obtained 
with great ease, either by calculation or by graphical con¬ 
struction, which latter is sufficiently accurate for these deter¬ 
minations. 


By these means we find, 


Degrees 
Forces. . 

Differences . 


13°, 14°, 15°, 16°, 17°, 18°, 19°, 20°, 21°. 
13, 14*1, 15*2, 16*3, 17*4, 18*6, 19*8, 21, 22*3. 
1*1, 1*1, 1*1, 11, 1% 1*2, 1*2, 1*3. 


Degrees 
Forces 
Differences . 


22°, 23°, 24°, 25°, 26°, 27°, 28°, 29°, 30°. 
23*5, 24*9, 26*4, 28, 29*7, 31*5, 33*4, 35*3, 37*3. 
1*4, 1*5, 1*6, 1*7, U8, 1*9, 1*9, 2. 


* That is to say, one reduced current is to the total current to which it corresponds 
as any other reduced current is to its corresponding total current. 



58 THE ABSORPTION AND RADIATION OF HEAT. 

In this table we do not take into account any of the degrees 
preceding the 13th, because the force corresponding to each of 
them possesses the same value as the deflection. 

The forces corresponding to the first 30 degrees being known, 
nothing: is easier than to determine the values of the forces 
corresponding to 35, 40, 45 degrees, and upwards. 

The reduced deflections of these three arcs are, 

15*3°, 22-4°, 29-7°. 

Let us consider them separately, commencing with the first. 
In the first place, then, 15 degrees, according to our calculation, 
are equal to 15*2; we obtain the value of the decimal 0*3 by 
multiplying this fraction by the difference 1*1 which exists 
between the 15th and 16th degrees; for we have evidently the 
proportion 

1 : 1-1 =0-3 : x = 0’3. 

The value of the reduced deflection corresponding to the 35th 
degree, will not, therefore, be 15*3°, but 15*2° + 0*3° = 15*5°. 
By similar considerations we find 23*5* o -f-0*6° = 24T o , instead of 
22*4°, and 36*7° instead of 29*7° for the reduced deflections of 40 
and 45 degrees. 

It now only remains to calculate the forces belonging to these 
three deflections, 15*5°, 24T°, and 36*7°, by means of the expres¬ 
sion 3*333 a; this gives us— 

the forces, 51-7, 80-3, 122*3; 
for the degrees, 35, 40, 45. 

Comparing these numbers with those of the preceding table, we 
see that the sensitiveness of our galvanometer diminishes con¬ 
siderably when we use deflections greater than 30 degrees. 


HISTORIC REMARKS ON MEMOIR I. 


Ir will lessen the labours of those who may hereafter wish to refer to this 
subject it I heie indicate the exact relationship, in point of time, of the foregoing 
investigation to the labours of a distinguished philosopher, now unhappily 
removed from us, who worked, as he stated, and as I believe, independently 
of me, on a portion of the area covered by these researches. 

1 lompted by the beautiful experiment of Grove, in which a white-hot 
platinum wire plunged into hydrogen gas is immediately chilled to darkness, 
the late Professor Magnus entered, in 18G0, on a skilful and ingenious experi¬ 
mental inquiry regarding the conduction of heat by gases. The most note¬ 
worthy lesult deduced by its author from this investigation is that hvdrogen 
conducts heat like a metal, and that the quenching of the glowing wire in 
Grove's experiment is a consequence of conduction. 

A preliminary notice (vorlaufige Mittheilung) of this investigation was com¬ 
municated to the Academy of Sciences in Berlin on July 30, 1800, being 
published in the ‘ Monatsberichte der Akademie’ for that date. 

Though the transmission of radiant heat through gases is not alluded to, nor 
even the term ‘ radiant heat ’ employed throughout this notice, still its subject 
and results naturally suggested the possible action of radiant heat. Accordingly 
we find Professor Magnus following up his first inquiry by another 1 On the 
Transmission of Radiant Heat through the Gases,’ which was communicated 
to the Academy of Sciences on February 7, 1861, and published in the 
‘ Monatsberichte ’ for that date. 

The two investigations here referred to appear together in extenso in Pog- 
gendorfi s 1 Annalen ’ for April 1861, and I had the pleasure of recommending 
them for translation in the ‘ Philosophical Magazine,’ where they appeared in 
July and August 1861. 

Professor Magnus’s memoir contains the following historic references 1c As 
far as I know, l)r. branz s are the only experiments which have hitherto been 
published on the diathermancy of gases. These, which moreover refer only to 
atmospheric air, hydrogen, and carbonic acid, could not be sufficient for the 
present purpose, because an argand lamp was used as a source of heat. But it 
was not merely possible, but probable, that the transmission of thermal rays 
would differ with their source. If, therefore, the experiments were to be con¬ 
clusive, the transmission must be investigated for rays proceeding from the same 
source of heat, that of boiling water.’ 

The paragraph preceding this one refers to 1860 as ‘ last year,’ hence it is to 
be inferred that this was written in 1861. 

In the next paragraph but one of his paper Professor Magnus makes the 
following passing reference to the relationship of his labours to mine :—‘ These 
experiments were already ended when I saw in the u Proceedings of the Roval 
Society ” that Dr. Tyndall in London is occupied with an investigation on the 
transmission of heat through gases. As Dr. Tyndall, whose investigation has 


60 


HISTORIC REMARKS OX MEMOIR I. 


been thus far only announced, lias subjected bis gases to experiment in tubes 
closed by plates of rock-salt, I believe tlie following results to be entirely in¬ 
dependent of the investigation of Mr. Tyndall.’ 

These two paragraphs occurring so nearly together, and yet so different, 
render it probable that the paper from which they are taken was written at 
different times, and that it was not until its later stages that Professor Magnus 
became aware of my researches. 

In the early part of 18G1 he wrote to me to inquire about the nature and 
course of my investigation, and I immediately sent him a sketch of my work. 
This was the origin of the following letter, which, to ensure perfect accuracy, 
I asked my friend Dr. Debus to translate for me soon after I received it. 
It clearly indicates the view then entertained by Professor Magnus cf cur 
respective labours:— 

Berlin : March 17, 1861. 

My dear Tyndall,—My apprehensions have indeed been realised. Both of us 
have been working on the same subject. I reproach myself for having been 
led into a mistake bj^ my consideration for you. In October of last year 
[ l.e . October 18(50], on occasion of the fiftieth anniversary of our University, 
and at a time when my experiments on the passage of radiant heat, using 
boiling water as a source of heat, had long been finished, Clausius told me that 
you were engaged with researches on the passage of radiant heat through 
gases. I confess the announcement of this investigation in the 1 Proceedings ’ 
had escaped my notice. I was doubtful if I should or should not write to you 
and communicate my results. I considered the case with Poggendorff, and it 
appeared to me that a communication of my results to you, who had been 
engaged on the subject for a whole year, would be a claim for priority. Also, 
as your notice appeared already in January, 18G0,* I supposed you had nearly 
or quite finished the investigation. I perceive now the better course would 
have been to have written to you, because Poggendorff tells me that your expe¬ 
riments were only made in November,! whilst the first part of my investigation, 
mentioned above, was already finished in July, when I was lecturing on the 
conduction of heat by gases. I believe you are sufficiently acquainted with me 
and my solicitude, to know that I should not have spoken on a conduction of 
gases if I had not convinced myself that the difference in the rise of tempera¬ 
ture in the same was not caused by a difference in the transmission of radiant heat. 
It was necessary for this reason to examine this transmission for the heat of boiling 
water which w r as used for conduction. The striking results obtained by me as 
well as by you made me desirous to examine, before publishing these results, 
how the gases would comport themselves with a source of heat of a high tem¬ 
perature, and this subject has occupied me during last autumn. It is now the 
third time that it has been my peculiar fate to have been working simultaneously 
with another person on the same subject, and also publishing the results at the 

* This ought to be May 1859, instead of January I860. The mistake may, perhaps, 
be thus accounted for. The ‘Philosophical Magazine’ usually gives a short account of the 
communications made to the Royal Society, and the number of that Journal for January 
1860 does contain a reprint of the communication made by me to the Royal Society eight 
months previously. Professor Magnus must have overlooked the heading of this section 
of the ‘ Philosophical Magazine,’ which runs thus:—‘May 26, 1859. Sir B. C. Brodie in 
the Chair. The following communications were read.’ 'I hen follows mine among others. 

f I do not understand this.—[J. T.J 


HISTORIC REMARKS OX MEMOIR I. 


61 


same time. The only satisfaction I derive from this circumstance is, that I 
had to compete with the most eminent experimentalists, such as Regnault and 
yourself. It has happened and cannot he altered. If we had more frequently 
corresponded it would not have occurred. Without doubt we both shall 
pursue the subject; let us therefore in future communicate to each other the 
special line of inquiry which each of us intends to follow, in order that we may 
not clash again. As soon as our’* papers are published, such an understanding 
can be more easily established than at present. You will have learnt from the 
February number of Poggendorff that my experiments do not extend to vapours, 
and the investigation of this part of the subject, excepting aqueous vapour, 
belongs exclusively to you. On the other hand, you have not examined 
ammonia, probably because it would have attacked your tube. This gas in¬ 
tercepts the rays of heat in a far higher degree than olefiant gas. On the 
radiation from gases no experiments have been made by me, and the inter¬ 
esting discoveries on this point are due to you alone. I intend to examine next 
the influence of reflecting plane surfaces, but before this I have to finish another 
investigation on the conducting power of gases for E., which also yields most 
remarkable results. Your statement in the ‘ Comptes Rendus,’ that moist air has 
a 15-times greater absorbing power for radiant heat than dry air, is not intelli¬ 
gible to me, except on condition that your air was cloudy or perhaps that a 
layer of moisture had settled on your rock-salt plates. For 1 was not able to 
notice a difference or perceptible difference between dry air and such which was 
saturated with vapour of water at 17° C. 

Heartily yours, 

G. Magxijs. 


To this letter I immediately sent the following reply:— 

My dear Magnus,—Though science is sure to be the gainer from our working 
independently at the same subject, it is not quite agreeable to find one’s own 
personal claims interfering with those of a valued friend. My simple course, 
however, is to lay before you a brief statement of the circumstances attending 
the investigation of the mutual action of gases and radiant heat, feeling assured 
that you will draw just conclusions from the statement. 

It is several years since the thought of examining the absorption of radiant 
heat by gases first occurred to me, but until the early part of 1859 I was so 
much occupied with other matters that I could not turn my attention specially 
to it. In 1859 I reflected a good deal on the question, made many experiments, 
devised new methods, and, after some weeks of successful work, communi¬ 
cated in a note to the Royal Society the mode of experiment and the general 
character of the results. This note, which was written with the express 
intention of setting my mind at rest as to the possible claims of other investi¬ 
gators, is published in the ‘Proceedings’ of the Society for May 26, 1859. 
I saw clearly that the subject would occupy me for several years. A new and 
vast field had in fact been laid open to experiment, and having by thought and 
labour fairly mastered it and obtained definite results, I felt at liberty to culti¬ 
vate it calmly and quietly without fear of being interfered with by anybody. 
In the note referred to I state distinctly that the investigation is in progress, a 
full account of it at a future day being promised to the Royal Society. 

I continued to work; and on June 10, 1859, gave a Friday evening discourse 
on the subject before the members of the Royal Institution. The Prince 


62 


HISTORIC REMARKS OK MEMOIR I. 


Consort was in the chair ; and by throwing the image of the dial of my galva¬ 
nometer upon a screen, I succeeded in making the experiments on the radiation 
and absorption of heat by gases and vapours visible to an assembly of many 
hundred persons. A report of the Lecture is published in the 1 Proceedings of 
the Royal Institution ’ for June 10, 1859. , 

You tell me that you had overlooked the notice in the ‘ Proceedings of the 
Royal Society,’ and that Clausius was the first to inform you in October 1860 
of my being engaged in experiments on the radiation of heat through gases. 
I, on the other hand, did not neglect to give my results due publicity. 
They were communicated to De la Rive in a letter published in the 
‘ Bibliotkeque universelle ’ for July 1859, vol. v. page 232. My lecture of the 
10th of June was also reproduced in ‘ Cosmos,’ vol. xv. page 321, where you 
will find the following words :—‘ Ces recherches de M. Tyndall sont encore a 
leur ddbut, il les continuera avec ardeur; deja il a perfectionne sa mode d’exp£- 
rimentation. Il emploie un galvanometre d’un seul fil, avec deux sources de 
chaleur agissant sur les deux faces d’une meine pile,’ &c. You will also find the 
subject referred to in the ‘ Nuovo Cimento,’ vol. x. p. 196. Also in the 
‘Comptes Rendus,’ and in other journals. When I was in Paris in 1859, my 
experiments formed a common topic of conversation among scientific men. 
Foucault, Jamin, Moigno, Wertheim and others, spoke to me about them. In 
fact they aroused an amount of attention far greater than I had any reason to 
expect. 

In London it was known to everybody that I was engaged on this question; 
Hofmann, \Y illiamson, and Frankland can all call to mind my obtaining volatile 
liquids from them in 1859. Faraday also recollects how often I have expressed ’ 
to him my satisfaction that anew field had been opened which I might cultivate 
at my leisure without fear of interfering with the claims of anybody. In 1859 
I had measured the absorption of oxygen, hydrogen, nitrogen, air, carbonic 
oxide, carbonic acid, nitrous oxide, coal gas, and olefiant gas; and I had also 
determined the absorption produced by the vapours of the following sub¬ 
stancesbisulphide and bichloride of carbon, sulphuric ether, cyanide of 
ethyl, chloroform, benzol, metliylic alcohol, amylene, iodide of ethyl, acetate of 
ethyl, propionate of ethyl, formate of ethyl, iodide of amyl, chloride of amyl, 
ethylic ether, ethyl amylic ether, amylic alcohol, absolute alcohol, and others. 
In fact in 1859 I had the materials for an elaborate memoir on this subject already 
in my hands. 

You say that I had not examined ammonia even in 1860. The fact, how¬ 
ever is that early in 1859 I had examined it and established its great absorptive 
power. If you will cast your eye on my letter to De la Rive published in the 
‘ Bibliotheque universelle ’ for July 1859, you will see that ammonia did not 
escape me.* 

Two circumstances offered a direct hindrance to the working out of my 

* The substances mentioned in my letter to M. De la Rive are ranged in the order of 
their absorptive power, which augments from the elementary gases through carbonic oxide, 
carbonic acid, nitrous oxide, olefiant gas, coal gas, up to ammonia. 

So also at page 47 of the foregoing memoir, which at the date (March 17) of Professor 
Magnus’s letter had been already more than two months in the possession of the Royal 
Society (handed in January 10), the following passage occurs‘ Thus we see that hydrogen 
and nitrogen, which when mixed together produce a small effect, when chemically united to 
form ammonia, produce an enormous effect.’ But again there is some ground for the 


HISTORIC REMARKS ON MEMOIR I. 


63 


results in 1859: the first was the desire to get the ‘ Glaciers of the Alps’ off 
my hands ; the second was a sudden call to undertake the duties of Professor at 
the School of Mines.* I resumed my experiments in I860, and you will find 
a general statement of my results at pp. 244-5 of the ‘ Glaciers of the Alps,’ a 
copy of which I had the pleasure of forwarding to Mrs. Magnus. Referring to 
the preface to that work, you will see that it was published in June 1860; and 
the preface was written several months subsequently to the pages above referred 
to. In fact the experimental notes now before me would satisfy you that in 
July 1859 I had obtained, in a substantially correct form, almost all the results 
of which you first heard in October 1860, and which you suppose I did not 
obtain until the November of that year. 

Believe me ever yours faithfully, 

John Tyndall. 

This small history, then, may be thus summed up:— 

On May 26, 1859, I communicated to the Royal Society a preliminary notice 
of an investigation ‘ On the Transmission of Radiant Heat through Gaseous 
Bodies;’ and on the 10th of June following I delivered before the Royal 
Institution a discourse ‘ On the Transmission of Heat of different qualities 
through Gases of different kinds,’ illustrating the subject by experiments. 

The former communication was published in the ‘Proceedings of the Royal 
Society,’ the latter in the ‘Proceedings of the Royal Institution’ for the respec¬ 
tive dates referred to. Soon afterwards notices and descriptions of the investi¬ 
gation appeared in various other journals both English and Continental. 

On July 30, 1860, or fourteen months after my first communication to the 
Royal Society, Professor Magnus communicated to the Academy of Sciences in 
Berlin a preliminary notice of an investigation on the Conduction of Heat by 
Gases. In this notice there is no mention of either radiation or absorption. 

After various and long-continued efforts to exalt the power and delicacy of 
my apparatus, to vary the source of heat, to bring clearly into view the action 
of the feeblest substances, and to confer by repetition quantitative certainty 
on the results, I communicated to the Royal Society on January 10, 1861, the 
foregoing memoir ‘ On the Absorption and Radiation of Heat by Gases and 
Vapours, and on the Physical Connexion of Absorption, Radiation, and Con¬ 
duction.’ 

My reasons for thus postponing publication are glanced at in pp. 14 and 15 of 
the foregoing memoir, where it is stated that during the summer of 1859 I had 
determined approximately the absorption of nine gases and of twenty vapours, 
having then in my possession the materials for a long memoir. Seven weeks 
of labour in 1859, and seven other weeks in 1860, are practically ignored 
through my reluctance to publish results which, though approximately correct, 
did not satisfy my desire for accuracy. 

On February 7, 1861, or about a year and three-quarters after my first, and 
about a month subsequent to my second communication to the Royal Society, 
Professor Magnus read before the Academy of Sciences in Berlin a memoir 

misapprehension of Professor Magnus ; because though I had established the position of 
ammonia in the list of absorbing gases, the apparatus I employed did not warrant me in 
assigning to it a numerical value. This 1 hastened to do in the very next inquiry. 

* I wished also to make my apparatus as perfect as possible. Indeed, having already 
secured myself by the wide publicity referred to, I was in no hurry to urge on my 
researches. 


64 


HISTORIC REMARKS ON MEMOIR I. 


entitled ( Ueber den Durchgang der Warmestrahlen durch die Gase/ It is 
stated to form tlie second part of an investigation the first part of which had 
been communicated to the Academy on the 30th ot July previously. 

Until I received the foregoing letter, dated from Berlin, March 17, I had no 
notion that Professor Magnus or any other investigator was engaged on this 
question. As stated by him in the letter just referred to, he had confined 
himself to the question of absorption, and he operated upon gases only. lie 
did not embrace radiation nor the action of vapours in his inquiry. 

I unreservedly accept his statement that he broke ground in this field inde¬ 
pendently of me, and that he was not aware until October 1860 that I had 
been engaged upon the subject. 

Were it correct, as supposed by Professor Magnus, that my experiments were 
not executed prior to November I860, I would willingly, on the mere strength 
of his assurance that his had been completed in July 1860, concede to him the 
priority which in his letter he seems to claim. I would not for a moment 
insist upon the circumstance that I had preceded him by a month in the 
publication of my investigation; but the fact is that results of far greater 
variety and number than those obtained by him were already in my possession 
in July 1859. In a subsequent paper samples of these results shall be produced. 
(See Memoir XII. p. 405.) 


♦ 


II. 

ON THE ABSORPTION AND RADIATION OF HEAT 

BY GASEOUS MATTER. 


5 


ANALYSIS OP MEMOIR II. 


♦ 


A 


The description of the instruments employed in the last investigation is here 
recapitulated, and the modifications of them which that inquiry suggested 
are described. The experiments on chlorine, ozone, and aqueous vapour are 
followed up. Chlorine is found to be far feebler as an absorbent than many light 
and transparent gases, while both ozone and aqueous vapour are found to exer¬ 
cise a far more powerful absorbent action than ordinary oxygen. Experiments 
on the human breath follow. Chlorine is then compared with hydrochloric 
acid and bromine with hydrobroinic acid, the lighter and more transparent 
compound gas being proved in each case more destructive of the heat-rays than 
the elementary one. 

The experiments with gases are extended, the gases being now enclosed 
in a glass experimental tube, and the source of heat changed from a cube of 
boiling water to a plate of copper, against which a constant sheet of flame is 
permitted to play. As in the last investigation, the elementary gases begin the 
list, as the lowest absorbers, while ammonia concludes the list, as the highest 
absorber. 

At the pressure of the atmosphere, ammonia is found to quench more than 
one thousand times the quantity of radiant heat intercepted by dry air. 

Smaller volumes of the gases are then compared together, the difference 
between the compound and elementary gases being thereby far more impres¬ 
sively brought out. At a common pressure of one inch of mercury, for example, 
the absorption of ammonia is proved to be over five thousand times, that of 
olefiaut gas over six thousand times, while that of sulphurous acid is six 
thousand four hundred and eighty times the absorption of dry atmospheric air. 
In this table chlorine and bromine, notwithstanding their colour and density, 
show themselves to be more diathermic than any of the transparent compound 
gases examined. 

Experiments on lampblack are next described; and the fact that this sub¬ 
stance exercises an elective absorption, acting differently upon different kinds of 
heat, is demonstrated. 

The experiments with vapours described in the last investigation are then 
followed up, and the extraordinary energy of absorption made manifest in 
Memoir I. is brought out with still greater emphasis. Some of the stronger 
vapours at a pressure of ^tli of an inch, or ^th of an atmosphere, are proved 
to exert GOO times the absorbent power of 30 inches of atmospheric air; and 
probably more than 180,000 times the power of air reduced to the same pressure 
as the vapour. 

It is pointed out that one gas or vapour may start, at a small pressure, with a 
lower power of absorption than another, and that when the pressure is gradually 
augmented, the one may overtake, and even transcend, the other. Further 
experiments on this point are contemplated. 

Following up an observation made in the last inquiry, several sections of this 





ANALYSIS OF MEMOIR II. 


67 


memoir are devoted to the dynamic radiation and the dynamic absorption of 
gases and vapours. An illustration or two will suffice to show the origin and 
character of these experiments. 

Alcohol vapour, under a pressure of half an inch of mercury, was admitted 
into the experimental tube : the consequent deflection was 72°. 

Dry air was permitted to enter by my assistant, while I, expecting only a 
slight augmentation of the deflection, was looking through the telescope at the 
needle. Instead of advancing, it fell to 0°, and swung to 25° on the opposite 
side of zero. 

A small modicum of ether vapour was admitted into the experimental tube : 
the deflection was 30°. On the entrance of dry air the needle fell to 0°, and 
swung to 60° on the other side. 

After the first moment of surprise and perplexity caused by this unexpected 
result, the explanation suggested itself that, as air is dynamically warmed on 
entering a vacuum, the reversal of the deflection might be due to the heat 
radiated from the vapour warmed by the air. If this were true, no external 
source of heat would be necessary to the production of the effect; but, on the 
contrary, an entirely novel method of determining radiant power would be 
available. This proved to be the case. The source of heat was abolished, and 
the dynamic radiation of vapours was determined by allowing a measured 
quantity of each to enter the experimental tube, and sending after it an atmo¬ 
sphere of air. These determinations harmonised perfectly with those obtained 
by the totally different methods described in the last memoir. 

Dynamic absorption was determined by chilling the mixed air and vapour 
by partial exhaustion, and permitting the pile to radiate its heat into the 
chilled vapour. Here, also, the results agree with those obtained by causing 
the vapours to intercept the heat from an external source on its wmy to the 
pile. 

The dynamic radiation and absorption of gases were also determined, and a 
similar agreement found to exist between them and former results. 

It is then shown that, by simply reasoning upon the physical conditions 
involved, w r e arrive at the conclusion that when a vapour exists under a small 
pressure in a long experimental tube, its dynamic radiation may exceed that 
of a gas which entirely fills the tube ; while when both columns are rendered 
sufficiently short, the radiation of the gas may far exceed that of the vapour. 
Experiment is shown to completely confirm this reasoning. 

It is worth noting that in the case of the sulphuric-ether vapour above 
referred to, which produced a deflection of 30°, a powerful flow of heat from a 
source of 212° Fahr. was at the time passing through the experimental tube to 
the pile. On the entrance of the air the vapour was raised five or six degrees 
in temperature, and this moderate amount of heating enabled the vapour not 
only to neutralise but to far overmatch, as a radiator, the selfsame vapour as an 

absorber of the heat emitted from the source. 

A section of the memoir is devoted to the action of odours upon radiant 

heat. 

A section is also devoted to the action of electrolytic oxygen, comprising its 
ozone. It is proved that the action on radiant heat augments as the size of the 
electrode diminishes, the action varying in the experiments recorded from 20 
to 13G. The harmony of this result with the totally different experiments of 
De la Rive and Meidinger is pointed out. The question whether ozone is a form 


68 


ANALYSIS OF MEMOIR II. 


of oxygen or a peroxide of hydrogen is subjected to an experimental test, the 
conclusion from which is, that ozone is produced by the packing together of 
oxygen atoms into molecular groups—that heat dissolves the bond of union 
between the atoms, and converts them into ordinary oxygen. 

Two sections are devoted to a comparison of the experiments on air, oxygen, 
hydrogen, and aqueous vapour with those of Professor Magnus. This dis¬ 
tinguished investigator, without any knowledge of what had been previously 
accomplished, had taken up a portion of this inquiry; and found the action 
of air, oxygen, and hydrogen to be much greater, and that of aqueous vapour 
much less, than my experiments make them. This whole question will be 
subsequently subjected to analysis. 

Experiments on the reduction of the temperature of a source by bringing air 
into contact with it are described. The action of an atmospheric envelope is 
again adverted to, and an experiment which offers a hope of determining the 
temperature of space is indicated. The memoir winds up by showing how the 
internal friction of air is affected by variations of density; the velocity of the 
atoms, caused by the same difference of temperature, being almost doubled when 
the density of the air is reduced to one-half. 


II. 


FURTHER RESEARCHES ON THE ABSORPTION AND 
RADIATION OF HEAT BY GASEOUS MATTER* 


§ 1 . 


Recapitulation. 


The apparatus made use of in this inquiry is the same in 
principle as that employed in my last investigation.f It grew uj) 
in the following way :—A wide tube was prepared for the gases 
through which radiant heat was to be transmitted ; but it was 
necessary to close the ends of this tube by a substance pervious 
to all kinds of heat, obscure as well as luminous. Rock-salt 
fulfils this condition; and accordingly plates of the substance 
an inch in thickness, so as to be able to endure considerable 
pressure, were resorted to. In the earliest experiments a cube 
of boiling water was placed before one end of this tube, and a 
thermo-electric pile connected with a galvanometer at-the 
other; it was found that if the needle pointed to any particular 
degree when the tube was exhausted, it pointed to the same 
degree when the tube was filled with air. By this mode of 
testing, the presence of dry air, oxygen, nitrogen, or hydrogen 
had no sensible influence on the radiant heat passing through 
the tube. 

. In some of these trials the needle stood at 80°, in some at 20°, 
and in others at intermediate positions. I, however, reasoned 
thus :—The quantity of heat which produces the deflection of 
20° is exceedingly small, and hence a minute fraction of this 
quantity, even if absorbed, might well escape detection. On 
the other hand, the quantity of heat which produces the de¬ 
flection of 80° is large, but then it would require a large 


* Eeceived by the Eoyal Society, January 9, and read before the Society, 
January 30, 1862. 

t Philosophical Transactions, 1861, and Philosophical Magazine , vol. xxii. p. 169. 


70 THE ABSORPTION AND RADIATION OF HEAT 

absorption to move tlie needle even half a degree in this position. 
A deflection of 20° is represented by the number 20, but a 
deflection of 80° is represented by the number 710. While 
pointing to 80, therefore, an absorption capable of producing a 
deflection equal to 15 or 20 of the lower degrees of the galvano¬ 
meter, would hardly produce a sensible motion of the needle. 
The problem then was, to work with a copious radiation, and 
at the same time to preserve the needle in a position where it 
would be sensitive to the slightest fluctuations in the absolute 
amount of heat falling upon the pile. 

This problem was first solved by the employment of a differen¬ 
tial galvanometer, and afterwards by converting the thermo-pile 
into a differential thermometer. Its second face was exposed, 
and a second source of heat was placed in front of that face. A 
moveable screen was interposed between the two, by the motion 
of which the same amount of heat could be caused to fall upon 
the posterior surface of the pile as that received from the 
experimental tube by its anterior surface. When this was 
effected, no matter how high the previous deflection might 
be, it was completely neutralized, and the needle descended 
to zero. 

The experimental tube being exhausted and the equilibrium 
established, it was immediately destroyed by the entrance 
of any gas, capable of absorbing even an extremely small 
fraction of the radiant heat. The second source predominated, 
and a deflection followed which, when properly reduced, became 
a strict measure of the absorption. 

But in these early experiments my radiating source stood at 
some distance from the anterior end of the tube, and the heat, 
previously to entering the latter, had to cross a space of air. 
This air and its possible sifting influence I wished to abolish, so 
as to allow the calorific rays to enter the gas with all the 
qualities which they possessed at the moment of emission. I 
first thought of soldering the end of the experimental tube 
direct to the radiating surface, thus allowing the gas to come 
into direct contact with the source. But it immediately occurred 
to me that the introduction of cool gas into the tube would lower 
the temperature of the source, and that it could never be known 
how far the indication of the galvanometer under such circum¬ 
stances could be regarded as a true effect of absorption. 


BY GASEOUS MATTER. 


71 


An independent tube, 8 inches long, and of the same diameter 
as the experimental tube, was therefore soldered on to the 
radiating plate. By a screw joint, the free end of this tube 
was connected air-tight with the experimental tube. Thus a 
chamber, from which the air could be removed, was introduced 
between the first plate of salt and the radiating surface, which 
was thereby withdrawn from all possible convective action. 
The radiant heat also entered the tube unchanged in quality 
save the infinitesimal change due to its passage through the 
diathermic salt. 


§ 2 . 


New Apparatus. 


I will ask permission to refer in this memoir to the Plate 
facing page 64: a verbal reference will in most cases be 
sufficient to indicate the changes recently introduced. S S', 
it will be remembered, represented the experimental tube, 
which was first made of brass polished within. Such a tube 
could not be used for gases or vapours capable of attacking 
brass and though this difficulty was combated by blackening 
the tube within, I could never feel at ease regarding the action of 
the gases upon the blackening substance. Many gases, more¬ 
over, present great difficulties on account of their affinity for 
atmospheric moisture. Hydrobromic and hydrochloric acid, for 
example, form dense fumes in the air; and however carefully 
they might have been dried, I should have been reluctant to 
base any inference on their deportment without actually having 
them under my eyes during experiment. 

The brass tube, then, which stretched from S to S' in the 
figure referred to is now replaced by one of glass, 2 feet 9 inches 
long, and 2*4 inches in diameter. The source of heat in my 
last-published inquiry was the cube of hot water C; but glass 
being far inferior to brass in reflecting power, I could not with 
this source bring out with the desired force the vast differences 
existing between various kinds Of gaseous matter. A hood of 
plate copper (mentioned at p. 14), was therefore employed. It 
was united by brazing to a tube 8 inches long, destined to form 
the vacuous chamber in front of the first plate of rock-salt. To 
lieat the copper plate, a lamp constructed on the principle of 





72 


THE ABSORPTION AND RADIATION OF HEAT 


Bunsen’s burner was made use of. The gas passed upwards by 
four hollow columns, each perforated for the admission of air. 
The mixture of air and gas issued into a chamber shaped like 
the frustrum of a cone, and over this chamber was placed a 
shade of thin sheet-iron, the top of which was narrowed to a 
slit one-eighth of an inch wide and 2 inches long. From this 
slit the mixture of gas and air issued, and formed upon ignition 
a sheet of flame. This was caused to glide along the back of 
the copper plate before referred to, which was thereby raised to 
a temperature of about 270° C. 

To preserve this source constant was one of the greatest 
difficulties of the investigation; for the slightest agitation of 
the surrounding air, or the least flickering of the flame itself, 
was sufficient to disturb the steadiness of the galvanometer and 
to render experiments in the most delicate cases impossible. 
The flame was therefore surrounded by screens of pasteboard, 
these being again encompassed by towels, through the meshes 
of which the flame was fed; a suitable chimney also thickly 
covered, produce a gentle draught and carried off the products 
of combustion ; the rhythmic jumping of the flame which sets 
in so readily was destroyed by screens of wire-gauze. The 
c compensating cube ’ Cf, the double screen H, and the thermo¬ 
electric pile P remain as before. They are exposed in the 
figure, but during the experiments they were surrounded by a 
close hoarding, all the chinks of which were stuffed with tow, 
so as to protect the cube and pile from the disturbing action of 
the air. The vacuous front chamber passed as before through 
a vessel V in which a current of cold water, constantly renewed, 
was caused to circulate. Six weeks’ practice was required to 
master all the difficulties of this portion of the apparatus. 

§3. 

Preliminary Efforts and Precautions. — Chlorine , Ozone , and 

Aqueous Vapour. 

On the 16th, 17th, and 18th of June, 1861,1 experimented on 
chlorine and on ozone, and satisfied myself that as an absorber 
of radiant heat, chlorine was far outstripped by many light 
colourless gases, and that ozone had a power of absorption very 
much greater than common oxygen. 


BY GASEOUS MATTER. 


73 


The work was resumed on the 12th of September, and my 
first care was to examine whether the experiments on moist 
and dry air described in Memoir I. stood the test of repetition.* 
Professor Magnus had found that the presence of aqueous 
vapour in air had no influence on the absorption of radiant heat; 
while according to my experiments dry air exercised only a 
small fraction of the absorptive energy of saturated air. I 
commenced my researches in September with a few experi¬ 
ments on this subject. 

Half an atmosphere of the undried air of the laboratory, ad¬ 
mitted directly into the experimental tube, cut off an amount 
of heat which produced a deflection of 30 degrees. 

The drying-apparatus at this time consisted of a IT-tube filled 
with fragments of pumice-stone wetted with sulphuric acid. 
Associated with this was a similar tube filled with like frasr- 
ments, but moistened with caustic potash in solution to remove 
the carbonic acid of the air. 

The air of the laboratory, passed through both of these tubes 
in succession, till a pressure of 15 inches was attained, gave a 
deflection of 26 degrees. 

This result surprised me, showing, as it seemed to do, a very 
close agreement between dr}’ and moist air. On examining the 
drying-tubes, however, it was found that, by a mistake of arrange¬ 
ment, the air had entered the sulpliuric-acid tube first, and 
passed straight from the potash into the experimental tube, 
thus partially reloading itself with moisture after it had been 
dried by the sulphuric acid. 

On causing the air to pass first through the potash, the 
deflection fell to less than 5 degrees. Hence this experiment 
showed the absorption due to the moisture and carbonic acid of 
the air to be more than six times as great as that of the atmo¬ 
sphere itself. It will presently be shown that this difference 
falls far short of the truth. 

The potash and sulphuric acid were now abandoned, the air 
being dried by passing it through a U-tube filled with fragments 
of chloride of calcium, which had lain in the tube for some 
months. The observed deflection was 40 degrees; that is to 
say, 10 degrees more than that produced by the undried air. 

This result, and many others of a similar nature, were due to 

* See Sections 20 to 22, Memoir II., and the whole of Memoir III. 





74 


THE ABSORPTION AND RADIATION OF HEAT 


tlie imperfection of the chloride of calcium. Chemists, I think, 
ought to be very cautious in the use of this substance as a 
drying agent. When freshly fused it answers well for this 
purpose, hut when old it yields an impalpable powder, which 
proved in the highest degree perplexing to me in my first 
experiments. It is generally found, I believe, that a drying- 
tube of sulphuric acid gains more in weight than one of chloride 
of calcium, and from this it lias been inferred that the quantity 
of moisture taken up by the former is greater than that taken 
up by the latter. The difference, however, may really be due to 
the mechanical carrying away of a portion of the chloride by 
the current of air. 

On the 13th of September these experiments were resumed. 
The dry air then gave a deflection of less than 2 degrees ; while 
the air direct from the laboratory caused, in one experiment, 
the needle to move from 20 degrees on one side of zero to 28 on 
the other. In a second experiment the undried air caused the 
needle to move from 18° on one side of zero to 32° on the other. 

Experiments made on the 17th entirely corroborated this 
result. Three successive trials with the undried air of the 
laboratory yielded the deflections 29°, 31°, and 30° respectively, 
while the deflection produced by the dried air was less than 
a single degree. On this day, therefore, the action of the 
aqueous vapour of the air was at least thirty times that of the 
air itself. 

Almost every week-day during the last four months, experi¬ 
ments similar to the above have been executed, and in no case 
have I observed a deviation from the result that the absorptive 
action of the aqueous vapour of the air is great in comparison 
with that of the air itself. Further on, this subject will receive 
additional illustration. 

As my mastery over the apparatus, and my acquaintance 
with the precautions necessary in delicate causes, increased, the 
absorption by air, and by the transparent elementary gases gene¬ 
rally, diminished more and more. I was induced to abandon 
the use of pumice-stone as well as chloride of calcium, and to 
construct the drying-apparatus in the following way. The 
internal portion of a massive block of pure glass was pounded 
to small fragments in a mortar; these were boiled in pure 
nitric acid, and afterwards washed several times with distilled 


BY GASEOUS MATTER, 


75 


water, so as to remove all trace of the acid. They were then 
dried, afterwards moistened with pure sulphuric acid, and in¬ 
troduced by means of a funnel into a U-tube. The funnel was 
necessary to preserve the neck of the tube from all contact with 
the acid, the least action of which upon the corks employed to 
close the tube being sufficient to entirely vitiate the experi¬ 
ments. At the top of each arm of the U-tube fragments of 
dry glass were placed, upon which any accidental dust or 
particles from the cork or sealing-wax might fall. 

Similar precautions were taken with the caustic-potash tube. 
Pure white marble was pounded to fragments and subjected to 
the action of a dilute acid, which removed the outer surface. 
The fragments were afterwards washed in distilled water 
and dried, then moistened with pure caustic potash, and in¬ 
troduced into the U-tube in the manner already described. 
It was sometimes necessary to perform this operation daily, and 
never on any occasion have I used tubes to dry a feeble gas 
which had been previously used to dry a powerful one. 

§ 4. 

First Experiments on the Human Breath.—Chlorine and 
Hydrochloric Acid.—Bromine and Hydrobromic Acid. 

In the present communication many subjects will be 
touched upon which for want of time I have been unable to 
develop. The following is an example of these. Choosing a 
day of suitable temperature and moisture—a day on wdiich the 
human breath shows no signs of precipitation—the action of the 
substances expired from the lungs may be determined. By breath¬ 
ing directly into the experimental tube, the action produced 
by the sum of the products of respiration might be accurately 
measured; by breathing through the sulphuric-acid tube, the 
moisture of the breath w T ould be withdrawn, and the difference 
between the action then observed and the former action would 
give that of the carbonic acid. In this way the products of 
respiration might be estimated singly, and the influence of 
various kinds of food and drink, or of physical exertion, on 
the respiration might be investigated in a manner hitherto 
unthouglit of. 

I have to record the following experiments only in connexion 


76 


THE ABSORPTION AND RADIATION OF HEAT 


with this point. Placing a suitable tube between my lips, and 
tilling my lungs with air, a stopcock which was interposed 
between me and the experimental tube was partially opened, 
and through it I breathed slowly into the tube until the mer¬ 
cury gauge of the pump was depressed 15 inches. I had, at 
the time, two assistants, C. A. and P. C., and they subsequently . 
breathed into the experimental tube the same quantity as 
myself. In the following table the absorption produced by 
the breath of each is stated ; the initials J. T. are my own :— 


Action of the Products of Respiration on Radiant Heat. 


Initials of 


Absorption 

Initials of 

Absorption 

person’s name 


per 100 

person's name 

per 100 

J. T. . 

• 

. 62 

J. T. ... 

. 59 

J. T. . 

• 

. 62 

R. C. ... 

. 63 

R. C. . 

• 

. 66 

C. A. . 

. 62 

R. C. . 

• 

. 68 

J. T. 

. . 60’5 

J. T. again . 

• 

. 59 



The absorption of dry air on 

the day that these 

results were 

obtained was found 

to 

be 1. 

The same dry air inhaled, under- 


went a chemical change which augmented its absorptive energy at 
least 60 times. This is given as a minor limit,; it is unnecessary 
to say how much I regard it as falling short of the truth. 

The day afterwards the following results were obtained, the 
same amount as before being exhaled :— 


Initials 

J. T. 
R. C. 
J. T. 
R. C. 


Absorption per 100 
. 56 

. 62 
. 56 

. 59 


In all cases P. C., who is the smallest and least robust man 
of the three, appeared to have the advantage. I will only add 
a few results obtained on the 6th of October, the quantity 
of air expired on the occasion depressing the mercurial column 
5 inches : — 


Initials 

J. T. 

R. C. 

R. C. After half a glass of Trinity Audit Ale 
Again ...... 

After a teaspoon ful of brandy 
After chewing and swallowing a small 
cf onion. 


Absorption per 100 
. . 33*5 

. . 35 

. 41 

. . 35 

. . 35 

quantity 

. 40 











BY GASEOUS MATTER. 


77 


After taking the ale and brandy my assistant washed his 
month and gargled his throat several times with cold water. 
I give these results merely as illustrative of one of the nume¬ 
rous applications of the apparatus. In all the experiments the 
tube remained perfectly transparent throughout, and, on pump¬ 
ing out, the needle in each case returned accurately to zero. 

In my last paper the fact was brought prominently forward 
that the elementary bodies then examined were far less hostile 
to the passage of the longer heat undulations than the com¬ 
pound ones. I was desirous this year to extend the experi¬ 
ments to one or two of the coloured gases and vapours, 
and on the 20th of September resumed the experiments on 
chlorine. This gas is highly coloured, and of a specific gravity 
of 2*45 ; one of its compounds, hydrochloric acid, is quite trans¬ 
parent, and of a specific gravity of only 1*26. Does the act of 
combination with hydrogen, which renders chlorine gas more 
transparent to light, render it also more transparent to heat ? 

Chlorine prepared from hydrochloric acid and peroxide of 
manganese, and dried by passing it through sulphuric acid, 
was admitted into the tube till it depressed the mercury gauge 
21 inches; the consequent absorption was expressed by the 
number 44. 

Hydrochloric acid was admitted till it depressed the gauge 
19 inches ; the absorption was 68. 

The following results were afterwards obtained : — 

Absorption per 100 


Chlorine 15 inches ... ... 32 

Chlorine 14 inches ....... 30 

Chlorine 14 inches ....... 30 

Hydrochloric acid 14 inches . . . . .47 

Chlorine again.30 

Hydrochloric acid ....... 56 


In all cases the effect of the compound gas was found to 
exceed that of the elementary one; so that the chemical change 
which renders chlorine more transparent to light renders it more 
opaque to obscure heat. 

I may remark that a subsidiary gauge was here used, so as 
to prevent the chlorine from entering the air-pump. 

Great care is required in experiments on hydrochloric acid, 
and great care was bestowed on the above. Previous to the 
introduction of the gas the experimental tube was filled with 





78 


THE ABSORPTION AND RADIATION OF HEAT 


perfectly dry air, so as to leave a perfectly dry residue on ex¬ 
haustion. The gas was allowed to stream through the drying- 
tube until all traces of air were expelled both from it and the 
retort; then a joint was suddenly broken, and the retort was 
connected with the experimental tube. The gas thus passed 
directly from the retort through the drying apparatus into the 
experimental tube. It was difficult to avoid sending in with 
the gas a few particles of moisture ; but these, if such existed, 
appeared to be dissipated by the dynamic heating of the gas on 
entering the tube, and kept in a gaseous state by the flux of heat 
passing through it. At all events the closest scrutiny could 
detect no trace of mist or turbidity within the tube; it was 
perfectly transparent throughout. The chlorine, on the con¬ 
trary, was intensely coloured. 

Many experiments were made with chlorine which had been 
collected over water, but something (what I know not yet) 
which materially augmented its absorption appeared to be in 
all cases carried along with the gas from the water into the 
tube. 

These experiments were made in the early part of the present 
inquiry, and before I had become aware of all the peculiarities 
of my apparatus. Subsequent efforts reduced in some degree 
the absorption both of chlorine and hydrochloric acid. Very 
careful experiments made on the 29th of October gave the 
following absorptions for these two gases, at a pressure of 30 
inches:— 

m 

Chlorine.39 

Hydrochloric acid.53 

After each experiment the chlorine and hydrochloric acid 
were removed from the experimental tube in the following 
manner:—A cock and connecting-piece were attached to one 
end of the experimental tube, and from them a length of india- 
rubber tubing led to the flue of the laboratory stove. A gas¬ 
holder of air was put in connexion with the other end of the 
experimental tube, a system of drying-tubes intervening be¬ 
tween the latter and the holder. By water-pressure a stream of 
dry air was forced gently through the experimental tube into 
the flue, and in this way the gases, which if pumped out would 
have injured the pistons, were speedily removed. As the dry 



BY GASEOUS MATTER. 


79 


air replaced the gases, the needle gradually descended to zero, 
its arrival there being indicative of the complete displacement 
of the gas. The perfect dryness of the air thus made use of 
was beautifully proved. Had the air contained moisture, it 
would instantly on its mixture with the hydrochloric acid have 
rendered the medium within the tube turbid; and however 
slight this turbidity might be, and however invisible to the eye, 
the galvanometer would have revealed it. But there was no 
movement in an upward direction; the needle gradually sunk 
from the moment the air entered. 

Bromine vapour and hydrobromic acid furnish another illus¬ 
tration of the influence of chemical union on the absorp¬ 
tion of radiant heat. The opacity of the former to light is far 
greater than that of chlorine, while the two compounds are 
equally transparent. The density of bromine vapour, moreover, 
is 5*54, whereas that of hydrobromic acid is only 2*75. The 
difficulty of operating with this acid is at least equal to that 
attendant on hydrochloric acid; and several successive days 
were spent in endeavouring to arrive at safe conclusions in 
connexion with this subject. Bromine dried with phosphoric 
acid was introduced into a flask furnished with a screw-cap, 
by which it was attached to the experimental tube. By 
turning a stop-cock, the pure vapour was allowed slowly to 
enter until the mercury column was depressed two inches. 
From more than twenty experiments made with this substance, 
I should infer that the absorption of the quantity mentioned 
does not exceed 11, while the absorption of hydrobromic acid 
of the same pressure amounts to 30. 

The hydrobromic acid was prepared by the action of glacial 
phosphoric acid (for a pure specimen of which I have to 
thank Dr. Frankland) on bromide of potassium. If the above 
figures represent the truth (and I have spared no pains to 
arrive at a right conclusion), we have here a most striking 
instance of transparency to light and opacity to obscure heat being 
p>romoted by the self-same chemical act* 

* A layer of liquid bromine, sufficiently opaque to intercept the entire luminous 
rays of a gas flame, is highly diathermanous to its obscure rays. An opaque solution 
of iodine in bisulphide of carbon behaves similarly. The details of these experiments 
shall be published in due time: they were publicly shown in my lectures many 
months ago.—June 13, 1862. 


80 


THE ABSORPTION' AND RADIATION OF HEAT 


§ 5 . 

New Experiments on Gases. 

. '> 

In the following table are given the absorptions of a number 
of gases at a common pressure of one atmosphere, as deter¬ 
mined with the new apparatus :— 

Table I. 

Name Absorption per 100 Name Absorption per 100 


Air . . , 

. 1 

Carbouic acifl . 

. 90 

Oxygen 

. 1 

Nitrous oxide . 

. 355 

Nitrogen . 

. . . 1 

Sulphuretted hydrogen 

. 390 

Hydrogen . 

. 1 

Marsh- gas 

. 403 

Chlorine . 

. 39 

Sulphurous acid 

. 710 

Hydrochloric acid 

. 62 

Olefiant gas 

. 970 

Carbonic oxide . 

. . 90 

Ammonia 

. 1195 


Air, oxygen, nitrogen, and hydrogen are all set down as 
equal to unity in the above table. I do not mean thereby 
to affirm that there are no differences between these gases, 
but that the most powerful and delicate tests hitherto applied 
have failed to establish a difference in a satisfactory manner. 
It is not improbable that the action of these gases may turn 
out to be even less than it is here found to be; for who can say 
that the best-constructed drying-apparatus is really perfect? 
Besides, stop-cocks must be greased, and hence may contri¬ 
bute an infinitesimal impurity to the air passing through them. 
It is not even certain that monohydrated sulphuric acid may 
not deliver a modicum of vapour to the current of air passing 
through it. At all events, if any further advance should be 
made in the purification of the gases, it will certainly only 
tend to augment the enormous differences exhibited in the 
above table. 

Ammonia, of the pressure mentioned, stands highest in the 
above list as regards absorptive energy. I believe a length 
of less than 3 feet of this gas, which to the vision is as trans¬ 
parent within the tube as the vacuum itself, to be perfectly black 
to the rays emanating from the source here made use of. When 
the gas was in the tube, the interposition of a double metallic 
screen between the pile and source augmented the deflection 
very slightly. But it will be shown, further on, that the 
ammonia in this experiment could not exhibit its full energy of 







BY GASEOUS MATTER. 


81 


absorption, and that in the length indicated it is in all proba¬ 
bility absolutely impervious to the heat issuing from our source. 

It would be a mere affectation of accuracy to try to deal with 
smaller quantities of the first four substances mentioned in the 
table than those here examined. Still, if such small quantities 
could be directly measured, the action of air, oxygen, hydrogen, 
and nitrogen, in comparison with that of the other substances 
at the same pressure, would doubtless be greatly reduced. With 
the energetic gases the rays are most copiously quenched by 
the portions which first enter the tube, the portions which enter 
last producing in many cases an infinitesimal effect. Now it 
has been shown in the last paper that, for very small absorp¬ 
tions, the effect is sensibly proportional to the quantity of gas 
present; and this would seem to justify the assumption that 
for 1 inch of pressure the absorption of air, oxygen, nitrogen, 
and hydrogen would be -^th of the absorption at 30 inches. 
In the case of each of the other gases I have measured directly 
the absorption of a quantity corresponding to a single inch of 
pressure. Assuming the proportionality just referred to, and 
again calling the effect of air unity (the unit, however, being 
only -gVth of that in the last table), the following are the 
relative absorptions:— 


Table II. 


Air 

. 1 

Carbonic oxide 

. 750 

Oxygen . 

1 

Nitric oxide . 

. 1590 

Nitrogen 

. 1 

Nitrous oxide. 

. 1860 

Hydrogen 

. 1 

Sulphide of hydrogen 

. 2100 

Chlorine 

. 60 

Ammonia 

. 5460 

Bromine 

. 160 

Olefiant gas . 

. 6030 

Hydrobromic acid . 

. 1005 

Sulphurous acid 

. 6480 


Here we have the extraordinary result, that, for pressures of 
1 inch of mercury, the absorption of ammonia is over five thousand 
times , the absorption of olefiant gas over six thousand times , while 
the absorption of sulphurous acid is six thousand four hundred 
and eighty times that of air. 

It is impossible not to be struck by the position of chlorine 
and bromine in this table. They are elements, and, notwith¬ 
standing their colour and density, they take rank after the 
transparent elementary gases. The perfectly transparent ole¬ 
fiant gas absorbs more than one hundred times the amount 
6 







82 


THE ABSORPTION AND RADIATION OF HEAT 


absorbed bj chlorine, and nearly forty times the quantity ab¬ 
sorbed by the intensely brown vapour of bromine. I cannot 
think this fact insignificant. Hitherto chemists have spoken to 
us of elements, and we have helped ourselves to conceptions 
regarding them and their compounds in the only way possible 
to our mental constitution. But our conceptions remained 
purely subjective, nor were we acquainted with any physical 
trait which would in any degree justify these conceptions. 
Here, however, we seem to touch the ultimate particles of 
matter. Starting from the idea that a gas absorbs such vibra¬ 
tions as are isochronous with its own, in all cases the compound 
gas reveals itself to the mind’s eye with its molecules swinging 
more slowly than the atoms of which it is composed, when un¬ 
combined. The absorption of the longer undulations proves 
the general coincidence in period with those undulations. We, 
as it were, load the atom by the act of chemical union, and 
thereby render its vibrations more sluggish, that is to say, more 
fit to synchronise with the slowly recurrent waves of obscure 
heat. 

In the foregoing table the absorption of nitric oxide is given 
as 1590, which is less than that of nitrous oxide, though 
the molecule of the former contains a greater number of atoms 
than that of the latter. It will be noticed that those gases which 
on combining suffer no condensation are less energetic absorbers 
than those which suffer a reduction of volume. Whether this rule 
is universal I am as yet unable to say. 

It is very difficult to operate with nitric oxide; the affinity of 
the gas for oxygen is so enormous that the slightest trace of 
this substance gives rise to the brown fumes of nitrous acid. 
On first sending this gas into the experimental tube, 1 inch of 
it gave an absorption of 2040 ; but the needle slowly went up 
afterwards, until it finally indicated an absorption of 5100. 
On looking across the tube at this time, the brown hue of 
nitrous acid was discernible. 

In a second experiment the vacuum was made as perfect as 
possible; and, on allowing nitric oxide to enter, the absorption 
was found to be 1860, but the needle soon afterwards declared 
an absorption of 3060, the brown fumes appearing as before. 

On filling the experimental tube with nitrogen, then exhaust¬ 
ing, and allowing nitric oxide to enter, the absorption of 1 inch 


BY GASEOUS MATTER. 


83 


of tlie gas was 1680. On filling the experimental tube pre¬ 
viously with hydrogen the absorption was 1590, which is that 
given in the table. On letting in a mixture of air and nitric 
oxide till the tube was filled, the action last mentioned was 
augmented nearly twentyfold. Nitrous acid is therefore an 
extremely energetic gas. The difference between it and bromine 
is enormous, even when their colours are undistinguishable. 

A close inspection of Melloni’s Table* reveals, I think, the 
tendency of solid bodies also to become more transparent to 
heat as their composition becomes more simple. After rock- 
salt itself, comes the element sulphur, and after it fluor-spar. 
But the case of lampblack will here occur to many, as the 
most powerful absorber and radiator yet discovered. No 
doubt the grouping of the atoms of an elementary substance 
may make it tantamount to a compound, and no doubt this is 
actually the case with lampblack; another eminent example 
of this kind is ozone. Leslie, however, found water to be a 
better radiator than lampblack, and Wells found several sub¬ 
stances which were more capable of being chilled by nocturnal 
radiation. On reflexion, moreover, the following considerations 
arise. The lampblack of commerce and the soot of a lamp or 
candle—that is to say, the substances hitherto employed in 
experiments on radiant heat—are copiously mixed with hydro¬ 
carbons, which are the most powerful absorbers and radiators 
in Nature. It might fairly be questioned whether the reputed 
experiments with lampblack really dealt with lampblack at all. 
But even the impure substance is to some extent transparent 
to radiant heat. 



Radiation through Black Glass and Lampblack. 

I have plates of black glass, rendered so by the solution of 
carbon in the glass while in a state of fusion, which, though 
impervious to the rays of the most intense electric-light, 
allow of a copious transmission of obscure heat. Melloni’s 
beautiful experiments on glass of this character are well 
known. Another of Melloni’s experiments which I have recently 
verified is the following. A plate of transparent rock-salt was 


* La Thermochrose, p. 164. 



84 


THE ABSORPTION AND RADIATION OF HEAT 


placed over a smoky camphine lamp, soot being deposited on its 
surface until it intercepted every ray of a brilliant jet of gas. 
The smoked plate was placed between a source of heat of a 
teuq^erature of 100° C. and a thermo-electric pile, a polished 
screen being placed between the salt and the source of heat. As 
long as the screen remained, the needle of the galvanometer 
connected with the pile stood at zero; but the moment the screen 
was removed the needle promptly advanced, showing the instan¬ 
taneous transmission across the layer of soot of a portion of the 
heat incident upon the salt. The actual numbers obtained 
in this experiment are these:—The deflection produced by the 
heat transmitted through the soot was 52 c ; which is equal to 
90 units. The deflection produced when the layer of soot had 
been carefully removed, so as to leave both surfaces of the salt 
smooth and transparent, was 71°, which is equal to 300 units. 
The quantity transmitted through the soot is therefore to the 
total quantity as 

90 : 300, 


or as 


30 : 100 ; 


that is to say, the lampblack, which was perfectly opaque to the 
light of a gas-jet, was transparent to fuUy 30 per cent, of the 
incident heat. On consulting Alelloni's Table, I was gratified 
to find that he made the transmission by a plate similarlv pre¬ 
pared 27 per cent.; while a layer so opaque that it cut off the 
beams of the sun itself transmitted 23 per cent, of the ravs 
emitted by a source heated to 100° C. 


§ 


Selective Absorption by Lampblack. 

At page 93 of c La Thermochrose/ Zffelloni examines the 
absorption of this substance for all sorts of ravs, and bv a series 
of ingenious experiments, and reasonings remarkable for their 
clearness and precision, he arrives at the conclusion that lamp¬ 
black absorbs with the same intensity aU description? of radiant 
heat.* At page 284, however, he cites and discusses with the 


* 1 Done, le noir de fumee absorbe avec la meme intensite toute sorte de ravonne- 
ments calorifiques’ (p. 101). 



BY GASEOUS MATTER. 


85 


same precision a series of experiments made with smoked rock- 
salt, in which he shows that the same layer of lampblack trans¬ 
mits 8 per cent, of the rays from a lamp of Locatelli, 10 per 
cent, of those of incandescent platinum, 18 per cent, of those 
from copper heated to 400° C., and fully 23 per cent, of those 
emitted by a source of 100° C. Now a transmission of 8 per 
cent, implies an absorption of 92; while transmissions of 10, 
18, and 23 per cent, imply absorptions of 90, 82, and 77. But 
that the self-same layer of lampblack absorbs 77 per cent, of 
the rays from one source and 92 per cent, of the rays from 
another, is at variance with the statement that lampblack 
absorbs heat from all sources with the same intensity. Suppose 
the surface of a thermo-electric pile to be coated by a layer of 
lampblack of the same thickness as that which coated Melloni’s 
plate of salt; 23 per cent, of the rays from a source of 100° C. 
would go right through such a layer and impinge upon the 
metal face of the pile; and if the latter were a good reflector, 
the heat incident upon it would be in great part retransmitted 
through the lampblack and lost to the instrument. For a source 
of 100° C., this loss would be many times greater than for a 
Locatelli lamp. Possibly, however, Mellon! may have meant 
simply to assert that for 'practical purposes the absorption at 
the face of his pile might be considered to be the same for all 
kinds of heat.* 


§ 8 . 

New Experiments on Vapours.—Further Proof of the Influence of 
Chemical Combination on the Absorption of Radiant Heat. 

I have now to record some new experiments on the action of 
vapours upon radiant heat. A number of glass flasks were 
prepared, of the shape and size of common test-tubes, each of 
which was furnished with a brass cap care fully cemented on to 
it. By means of this it could be attached to a stopcock, and 
thus connected with the experimental tube. The mode of 
operation was this:—The liquid was introduced into the flask 


* The sun, through the floating carbon of the London atmosphere, sometimes 
present's a most instructive appearance. Entirely shorn of his rays and of perfectly 
uniform brightness, his colour at times is as red as blood. This is doubtless in part 
due to the comparative transparency of the floating carbon for the longer undulations. 





86 


THE ABSORPTION AND RADIATION OF HEAT 


by means of a small glass funnel. A stopcock was then attached 
to the flask and connected with a second air-pump, which was 
always kept at hand. The air above the liquid was removed, 
and the air dissolved in it was allowed to bubble away, until 
nothing remained but the pure liquid below and the pure vapour 
above it. The stopcock was then shut off, and the flask united 
to the experimental tube. The exhaustion of the tube being* 
complete, and the needle of the galvanometer at zero, the cock 
attached to the flask was turned on and the mercury-gauge 
carefully observed at the same time. No bubbling of the 
•liquid was in any case permitted. The vapour entered silently 
and without the slightest commotion ; and when the mercurial 
column was depressed to the extent required, the vapour was 
promptly intercepted. 

The energy with which the needle moves the moment a 
strong vapour enters is so extraordinary> that, lest the 
shock against them should derange the magnetism of the 
astatic pair, I removed the stops which arrested the swing of 
the needle at 90°. It often swung far beyond a quadrant; 
and after it had come finally and permanently to rest, 
its position was observed in the following manner:—The 
dial of the galvanometer being well illuminated, a looking- 
glass was placed behind the instrument, at such an angle 
that when looked at horizontally the image of the dial was 
clearly seen. This image was observed by an excellent tele¬ 
scope fixed at a distance of 11 feet from the galvanometer. 
Attached to the needle, and in continuation of it, was a bit of 
glass fibre of extreme fineness, blackened with Indian ink. 
This index ranged over the graduated circle, and by means of it 
a very small fraction of a degree could be easily read off. The 
expedient of observing from a distance was resorted to, because 
it was found that my approach to the galvanometer, perhaps 
through the diamagnetic action of my own body, had a sensible 
effect upon the needle, which, I believe, surpasses in delicacy 
any hitherto employed. 

The permanent deflection of the needle was noted in all these 
experiments, and the value of the deflection, expressed in terms 
of one of the lower degrees of the galvanometer, was obtained 
from a table of calibration. To spare unnecessary labour 1 
omit the deflections in the following table, and give the 


BY GASEOUS MATTER. 


87 


absorptions only produced by the vapours, at 0*1, 0*5, and 1*0 
inch of pressure. 

Table III. 


Pressures 


Name of substance 


t - 

0'1 inch. 

0 5 inch 

1*0 inch 

Bisulphide of carbon 

• 

. 15 

47 

62 

Iodide of methyl . 


. 35 

147 

242 

Benzol 


. 66 

182 

267 

Chloroform . 


. 85 

182 

236 

Iodide of ethyl 


. 158 

290 

390 

Methylic alcohol . 


. 109 

390 

590 

Amylene . . . 


. 182 

535 

823 ‘ 

Alcohol 


. 325 

622 


Sulphuric ether 


. 300 

710 

870 

Formic ether 


. 480 

870 

1075 

Acetic ether . 


. 590 

980 

1195 

Propionate of ethyl . 


. 596 

970 


Boracic ether 

• 

. 6 20 



Let us compare some 

of the results of this 

table of trans- 


parent vapours with the action of the highly coloured vapour of 
bromine. The absorption of bromine vapour at 1 inch pressure 
is about 6, and at 0T of an inch pressure would probably not 
exceed 1; hence at 0T of an inch pressure, bisulphide of carbon 
exerts probably 15 times the absorbent power of bromine ; but 
bisulphide of carbon is the feeblest of the compound vapours 
hitherto discovered. The strongest of these, boracic ether, 
has, according to the above estimate, and at the pressure stated, 
more than 600 times the absorbing energy of the strongly coloured 

bromine. 

The whole of the numbers in the above table are referred to 
atmospheric air as unity; 0T of an inch of bisulphide-of-carbon 
vapour, for example, absorbs 15 times as much as a whole 
atmosphere of air. Let us compare, for an instant, the action 
of boracic ether with that of air. We arrive at an approximate 
comparison in this way. The absorption of the tenth of an inch 
of boracic ether is something more than that of a whole inch 
of methylic alcohol; by diminishing the quantity of methylic 
alcohol to one-tenth, we reduce its absorption from 590 to 109. 
The absorption of one-tenth of an inch of boracic ether is 620°; 
suppose its absorption to diminish with diminished quantity in 
the proportion of methylic alcohol, we should then have for 0‘01 
of an inch of boracic ether an absorption of 111; that is to say, 









88 


THE ABSORPTION AND RADIATION OF HEAT 


for 3 "oVoth of an atmosphere of boracic ether, we should have an 
action 111 times that of a whole atmosphere of oxygen, nitrogen, 
hydrogen, or atmospheric air. 

With the transparent elementary gases it is impossible to 
measure directly the absorption of 0-1 of an inch; but assuming, 
as before, that up to an absorption of 1 the effect is proportional 
to the quantity of gas present, the absorption of each of the ele¬ 
mentary gases, at a pressure of 0*1 of an inch, would be about 
0*0033 ; hence the absorption of boracic ether of 0*1 of an inch 
pressure is to that of air at the same pressure as 

620 : 0-0033, 

which would give to the ether an energy 186,000 times that of air . 

I have already spoken of the blackness of ammonia at 30 
inches pressure. Beferring to Table I., its absorption is found 
to be 1195. In the last table the vapour of acetic ether, 
under only one-thirtieth of the pressure of the ammonia, 
produces apparently the same effect; its absorption is also 
1195. Such facts give one entirely new ideas of the capabilities 
of matter ; and our wonder will not be diminished by the results 
to be recorded further on. 


§ 0 . 

Superior A.ction at one Pressure does not prove Superiority 

at all Pressures . 

With both gases and vapours we find that it does not follow 
that a gas which produces a larger effect than another at one 
pressure should surpass it at all other pressures. Some gases 
start from a lower level than others, but finally attain an equal, 
or even a greater elevation. If their absorptions were represented 
by curves plotted from the same datum-line, these curves would 
in some cases approach, and in some cases, cross each other. At 
a pressure of 1 inch, for example, carbonic acid has more than 
double the absorptive power of carbonic oxide, whereas at a 
pressure of 30 inches they are equal; indeed some of my ex¬ 
periments show carbonic oxide to have the advantage. On the 
22nd of October, for example, the deflection produced by 2 inches 
of carbonic oxide was found to be 15°, while that of 2 inches of 


BY GASEOUS MATTER. 


89 


carbonic acid was 38°. Tlie two gases at a pressure of 30 inches 
gave these results :— 

Degrees 

Carbonic oxide . . . .52 

Carbonic acid. 51*5 

And again, on the 4th of November I obtained the following 
relative effects:— 

Pressures 

t -;- A -:-\ 

1*2 inch 24 inches 

Carbonic oxide .... 12 5°7 

Carbonic acid .... 37 54 

The same remarks apply to vapours. Methylic alcohol, for 
example, starts at a lower level than the iodide of ethyl, but 
ascends more quickly, and finally reaches a much higher eleva¬ 
tion. The same observation applies to benzol and the iodide 
of ethyl, in comparison with chloroform. 

§ io. 

DYNAMIC RADIATION AND ABSORPTION. 

A class of facts are now to be referred to which surprised and 
perplexed me when I first observed them. As an illustration, 
I will take the case of alcohol vapour. A quantity of this sub¬ 
stance, sufficient to depress the mercury gauge 0*5 of an inch, 
produced an absorption which caused a deflection of 72° of the 
galvanometer needle. 

TV hile the needle pointed to this high figure, and previously 
to pumping out the vapour, dry air was allowed to stream into 
the experimental tube, and I happened while it entered to observe 
the effect upon the galvanometer. The needle, to my asto¬ 
nishment, sank speedily to zero, and went to 25° at the opposite 
side. The entry of the almost neutral air here not only abolished 
the absorption previously observed, but left a considerable balance 
in favour of the face of the pile turned towards the source. A 
repetition of the experiment brought the needle down to zero, 
and sent it to 38° on the opposite side. In like manner a very 
small quantity of the vapour of sulphuric ether produced a deflec¬ 
tion of 30°; on allowing dry air to fill the tube the needle de¬ 
scended speedily to zero, and swung to 60° at the opposite side. 

These results both perplexed and distressed me, imagining 





90 THE ABSORPTION AND RADIATION OF HEAT 

as I did, on first observing them, that I had been dealing 
throughout with an effect totally different from absorption. I 
thought, indeed, that the vapours had deposited themselves in 
opaque films on the plates of rock-salt, and that the dry air on 
entering had cleared these films away, and allowed the heat from 
the source free transmission. 

But a moment’s reflexion dissipated this supposition. The 
clearing away of such a film could at best but restore the state 
of things existing prior to its formation. It might be conceived 
of as bringing the needle again to 0°; but it could not possibly 
produce the negative deflection, which, in the case of ether- 
vapour, amounted to the vast amplitude of 60°. Nevertheless 
I dismounted the tube, and subjected the plates of salt to a 
searching examination. No such deposition as that above 
surmised took place. The salt remained perfectly transparent 
while in contact with the vapour. 

Some of the experiments recorded in the Bakerian Lecture 
for 1860 had taught me that the dynamic heating of the air 
when it entered the exhausted tube was sufficient to produce 
a very sensible radiation on the part of any powerful vapour 
contained within the tube, though I was slow to believe that 
the enormous effect now under consideration could be thus 
accounted for. My first care was to determine the difference 
of temperature within the experimental tube at the end furthest 
from the source of heat, and the air without. I then examined, 
by an extremely sensitive thermometer, the increase of tem¬ 
perature produced by the admission of dry air into the tube, and 
the decrease of temperature consequent on pumping out, and 
found the former to be a considerable fraction of the total heat 
transmitted from the source. Could it be that the heat thus 
imparted to the alcohol and ether vapours, and radiated by them 
against the adjacent face of the pile, was more than sufficient to 
make good the loss by absorption? The experimentum crucis 
at once suggested itself. If the effects observed were due to 
the dynamic heating of the air, we ought to obtain them even 
when the sources of heat are entirely abolished. We should 
thus arrive at the solution of the novel, and at first sight utterly 
paradoxical problem, dealt with in the next section. 








BY GASEOUS MATTER. 


91 


§ 11 . 

To determine the Radiation and Absorption of Gases and Vapours 
without any Source of Heat external to the Gaseous Body itself. 

I.— Vapours. 

For the sake of brevity, I will call the heating of a gas on its 
admission into a vacuum, the dynamic heating of the gas; and 
the chilling accompanying its pumping out, dynamic chilling. 
It would also contribute to brevity if I were allowed to call the 
radiation and absorption of the gaseous body, consequent on 
such heating and chilling, dynamic radiation and dynamic 
absorption , though the terms are not unobjectionable. 

Both the source of heat and the compensating cube were 
dispensed with, and the thermo-electric pile was presented to 
the end of the cold experimental tube. By a little manage¬ 
ment, the slight inequality of radiation against both faces of 
the pile, arising from differences of temperature in the various 
parts of the laboratory, was obliterated, and the needle of the 
galvanometer was brought to 0°. 

The vapours were admitted in the manner already described, 
until a pressure of 0*5 of an inch was obtained. The air was 
then allowed to enter through a drying-apparatus by an orifice 
of a constant magnitude. Two stopcocks, in fact, were intro¬ 
duced between the drying-tube and the experimental tube; one 
of these was kept partially turned on, and formed a gauge for 
the admission of the air. When the tube was to be exhausted, 
the second stopcock was turned quite off. When the tube was 
to be filled, this stopcock was turned full on; but the gauge-cock 
was never touched during the entire series of experiments. 

Before, however, the mode of experiment was thus strictly 
arranged, a few preliminary trials gave me the following re¬ 
sults :— 

Nitrous oxide on entering caused the needle to swing in a 
direction which indicated the heating of the gas; the limit of 
its excursion was 28°, after which it slowly sank to 0°. 

The pump was now worked ; the propulsion of the first por¬ 
tions of the gas from the tube was so much work done by the 
residue. That residue became consequently chilled; into it the 
adjacent face of the pile poured its heat, and a swing of the 


92 


THE ABSORPTION AND RADIATION OF HEAT 


needle on the negative side of 0° was the consequence. The 
limit of the excursion was 20°. 

Olefiant gas, operated on in the same manner, produced on 
entering the tube a swing of 67°, showing radiation; and on 
pumping out, a swing of 41°, showing absorption. After the 
pumping out of the gas, and without introducing a fresh quan¬ 
tity, dry air was again admitted; the swing produced by the 
dynamic radiation of the residue of the gas (0’2 of an inch in 
pressure) was 59°. On pumping out very quickly, the dynamic 
absorption produced a deflectiou of nearly 40°. 

A little of the vapour of sulphuric ether was admitted into the 
tube; on the admission of dry air afterwards, the needle swung 
from 0° to 61°; on pumping out, the needle ran up to 40° on 
the opposite side. 

These and other experiments, which gratified me exceed- 
ingly, showed that, without resorting to any source of heat 
external to the gaseous body itself, its radiation and ab¬ 
sorption might be determined with extreme accuracy, and 
the reciprocity of both phenomena rendered strikingly clear. 
In fact, at this very time I had been devising an elaborate 
apparatus for the purpose of examining the radiation of gases 
and vapours, with a view to comparison with their absorption; 
but no such apparatus would have given me results equal in 
accuracy to those placed within reach by the discovery here 
referred to. 

The following table is the record of a series of experiments on 
dynamic radiation and absorption. The vapour in each case 
was admitted till the mercury column fell half an inch, and 
dry air was admitted afterwards. 

Table IY.— Dynamic Radiation ancl Absorption of Vapours. 

Deflections 



Radiation 

Absorption per 100 

Bisulphide of carbon .... 

O 

. 14 

O 

6 

Iodide of methyl ..... 

. 19-5 

8 

Benzol. 

. 30 

14 

Iodide of ethyl ..... 

. 34 

, 15*5 

Methylic alcohol. 

. 30 


Chloride of amyl ..... 

. 41 

23 

Amylene ...... 

. 48 

Alcohol ...... 

. 50 

27'5 

Sulphuric ether . 

. 64 

34 

Formic ether . 

. 68-5 

38 

Acetic ether . 

. 70 

43 








BY GASEOUS MATTER. 


93 


The paradox already referred to is here solved, and an expla¬ 
nation given of the extraordinary effect observed in the case of 
the alcohol and ether vaponrs when dry air entered the experi¬ 
mental tube. Dynamic radiation, moreover, and dynamic ab¬ 
sorption go hand in hand; and if we compare both with Table 
III. (middle column), we shall find the order of the substances 
precisely the same, although one set of results are obtained 
with a source of heat external to the gaseous body, and the 
other with a source of heat and cold within the body itself. 
Were sufficient time at my disposal, this subject could be 
developed with advantage. The measurements just recorded 
constitute my first regular series; and, no doubt, augmented 
experience will enable me to attain more perfect results. 

Half an inch of my most energetically acting vapour— 
namely, boracic ether—could not well be obtained ; but one- 
tenth of an inch admitted into the tube and dynamically heated 
and chilled, gave— 

Radiation Absorption per 100 

56° 28° 



Attempted Estimate of Quantity of Radiant Vapour. 


Seeing the astonishing energy with which some of these 
vapours absorb and radiate heat, it may be asked how far the 
quantity of vapour may be reduced before its action becomes 
insensible. At present I will not venture to answer this question 
fully; certainly we should be dealing at least with millionths 
of our smallest weights. But I will here give a detailed 
account of one experiment, the result of which can hardly 
fail to excite surprise. The experimental tube being ex¬ 
hausted, one-tenth of an inch of boraeic-ether vapour was 
admitted into it: the barometer stood at 30 inches at the time; 
hence the pressure of the vapour within the tube was yj-yth 
of an atmosphere. 

Dynamically heated by dry air, the radiation of this vapour 
produced a deflection of 56°. 

The tube was then exhausted to 0-2 of an inch, and the quan¬ 
tity of vapour reduced thereby to jA-tli P ar t °f its first 
amount ; the needle was allowed to come to zero, and the 



94 


THE ABSORPTION AND RADIATION OF HEAT 


residue of vapour was dynamically lieated as before : its radia¬ 
tion produced a deflection of 42°. 

The pump was again worked till a vacuum of 0*2 of an inch 
was obtained, this residue containing of course yi-ytli of the 
quantity of ether present in the last. On dynamically heating 
this residue, its radiation produced a deflection of 20°.* 

Two additional exhaustions, succeeded by dynamic heating, 
gave the deflections 14° and 10° respectively. 

Tabulating the results so as to place each deflection beside 
the vapour-pressure which produces it, we have the following 
view of the experiment: — 


Table Y .—Dynamic Radiation of Boracic Ether. 

Deflection 


Pressure in parts of 
an atmosphere 


i 

iso 

l 


l 

150 

1 

15 0 


_, y 1 - y _i_ y_ 

150 150 * 150 300 


1 _ 

_i_th 

300 

1 th 

300 

45000 LIA 

i 

i i 

300 

6750000' 

1 

i 


1012500000 


th 

™th 


O 

56 

42 

20 

14 


The air itself, slightly warming the apparatus near the pile, 
produces a feeble radiation, amounting to 6° or 7°. I have pur¬ 
posely excluded the deflection 10°, in order to show that the 
effect was still diminishing when the experiment ended, the con¬ 
stant effect due to the air itself being not yet attained. Two Os are 
thus excluded from the denominator of the fraction which might 
fairly have appeared in it. The above result is, however, suffi¬ 
ciently extraordinary, showing as it does that the radiation of 
an amount of vapour possessing in the experimental tube a 
pressure of less than the thousand-millionth of an atmosphere is 
perfectly measurable. It will also be borne in mind that the 
temperature imparted to this infinitesimal quantity of matter 
could not be high.f 


These experiments, which I intend to develop on a future 
occasion, seem to give us new ideas as to the nature and capa- 

* This is less than the truth—my assistant having executed three or four strokes 
of the pump inadvertently while the dry air was not shut off, removing thereby a 
considerable proportion of the vapour which ought to be present at this stage of the 
experiment. 

t I should like to repeat these experiments on boracic ether for this reason : the 
liquid, when it evaporates in moist air, leaves a solid residue of boracic acid, which may 
be seen round the stopper of the bottle containing the liquid; and, though it is not prob¬ 
able that any such residue was formed in the foregoing experiments, I should like to 
re-examine the vapour with special reference to this point. 





BY GASEOUS MATTER. 


95 


bilities of matter. A platinum wire heated to whiteness in a 
vacuum by an electric current, becomes comparativelycold within 
a second after the current has been interrupted ; yet that wire, 
while ignited, was the repository of an immense amount of 
mechanical energy. What has become of this ? It has been 
conveyed away by a substance so attenuated that its very exis¬ 
tence must for ever remain a hypothesis. But here is matter 
that we can weigh, measure, taste, and smell, proved to be 
reducible to a tenuity which, though expressible by numbers, 
defies the imagination to conceive it. Still we see it com¬ 
petent to arrest and originate quantities of energy which in 
comparison with its own mass must be almost infinite, a small 
fraction of this energy causing the double needle of the gal¬ 
vanometer to swing through considerable arcs. When we find 
common ponderable matter producing these effects, we have 
less difficulty in investing the luminiferous aether with those 
mechanical properties which have long excited the interest and 
wonder of those who have dwelt on the mechanical conceptions 
involved in the undulatory theory of light. 

§13. 

II.— Gases. 

In the foregoing experiments dry air was used to warm the 
vapours, but similar differences ought to be exhibited by gases 
when heated by their own dynamic action. That this is the 
case the following experiments show :— 

Table YI .—Dynamic Radiation of Gases. 

Name Radiation 


,Air..... 

.7 

Oxygen .... 

.7 

Nitrogen. . . . 

.7 

Hydrogen . . . 

.7 

Carbonic oxide 

.19 

Carbonic acid . 

.21 

Nitrous oxide . 

.31 

Olefiant gas 

.63 


These results are in accordance with those recorded in 
Table I., p. 80. 

The following two gases were used in irregular quantities, but 
the energy of their radiation is thereby established beyond a 
doubt. They were admitted into the experimental tube from 









THE ABSORPTION’ AND RADIATION OF IIEAT 


96 


a large boltliead, until a common pressure was established be 
tween the gas in the tube and the gas in the boltliead. 


Ammonia 15 in. pressure 
Sulphurous acid 16 in. pressure 


Badiation 


. 5 6 ‘ 5 
. 45 


Absorption per 100 


335 


24 


§ 14. 

Influence of Length and Density of Radiating Column. 

Let us reflect for an instant on the condition of our tube, con¬ 
taining its -J-inch of vapour, at the moment when the latter has 
been heated by the entrance of the air. The rays from the 
molecules at the end of the tube most distant from the pile have 
to cross a space of nearly 3 feet before they reach the adjacent end, 
this space being filled with molecules similar in all respects to 
the radiating ones. Hence absorption to a comparatively great 
extent must occur; and indeed we can imagine the tube so 
long that its frontal portion should furnish a vapour screen 
absolutely opaque to the radiation of its hinder portion. Now 
comparing ether-vapour with olefiant gas, it is, I think, evident 
that the radiant points of the attenuated vapour, which de¬ 
presses the mercury column only 0*5 of an inch, are further 
apart than those of the gas which depresses the column 30 
inches. Consequently there is a wider door open for the radi¬ 
ation of the distant ether particles towards the pile than for 
the distant particles of olefiant gas. The length of the whole 
column, in fact, might be more or less available for the radiation 
of the vapour, and a part of it only available for the gas. Cut off 
this useless portion from the gas column, and we do not injure 
its efficacy; but cut off a similar length from the vapour column, 
and we may materially diminish its effect. Speaking generally, 
in reducing the column of ether and that of gas by the same 
amount, the diminution of radiation will be more sensibly felt 
where the radiant points are furthest asunder. Reasoning 
thus, it becomes evident that in a long tube the vapour may 
excel the gas in its amount of radiation, while in a short 
tube the gas may excel the vapour. Let us now test this 
reasoning by experiment. 

The dynamic radiation of the following four substances has 
been tabulated thus :— 


BY GASEOUS MATTER. 


97 




Sulphuric ether 
Formic ether 
Acetic ether. 
Olefiant gas. 


Degrees 
. 64 

. 68*5 
. 70 

. 63 


The action of olefiant gas is therefore smallest when the length 
of the radiating column is 2 feet 9 inches. 

With a tube 3 inches long, or one-eleventh of the former 
length, precisely similar experiments yielded the following 


results:— 



Degrees 

Sulphuric ether . 

. 11 

Formic ether 

. 12 

Acetic ether 

. 15 

Olefiant gas 

. 39 


The verification of the above theoretic reasoning is here com¬ 
plete. It is proved that in a long tube the dynamic radiation of 
the vapour exceeds that of olefiant gas, while in a short tube the 
dynamic radiation of the gas far exceeds that of the vajpour . 


§ 15 . 

Laplace 9 s Correction for the Velocity of Sound.—Remarks on the 
Radiant Power of Molecules and Atoms. 

Some years ago a discussion was carried on between Pro¬ 
fessors Challis and Stokes on Laplace’s correction for the 
velocity of sound in air. Professor Challis contending that 
Laplace had no right to his correction, inasmuch as the heat 
developed in the condensations of the waves of sound would be 
instantly wasted by radiation. Experiments, he argued, con¬ 
ducted in confined vessels furnish no ground for conclusions 
regarding what occurs in the atmosphere, where the heat 
developed has an indefinite space to lose itself in. Now, our 
experimental tube, though mechanically closed, is thermally 
open ; by employing the rock-salt plate, indefinite extension, 
as regards the radiation of heat, is secured in one direction, 
and the means also exist of measuring the flux of this 
heat. What is true for one direction would of course be 
true for all, so that the apparatus will inform us of what 
occurs in the open atmosphere. The fact, then, is that, 
with the most powerfully radiating gases hitherto examined, 

r 


98 


THE ABSORPTION AND RADIATION OF HEAT 


tlie radiation continues a very sensible time, while the heat 
acquired by air, on entering the tube, is often a source of 
inconvenience on account of the inability of the air to disperse 
its heat by radiation. The question seems therefore experi¬ 
mentally decided in favour of Laplace and his supporter. 

This lack of radiating power on the part of air, and of the ele¬ 
mentary gases generally, is very noteworthy. The dynamically- 
warmed air is the proximate source of the heat imparted to the 
vapours in our experiments on dynamic radiation. It is related 
to those vapours precisely as a hot plate of polished metal to the 
coat of varnish which makes it a radiator. TV ithout the inter¬ 
mediation of a second body neither the air nor the metal (both 
of them elements or mixtures of elements) is competent to 
impart motion to the luminiferous aether. The atoms possess 
the motion of heat, but they cannot communicate it to the 
light-medium, save in the scantiest degree. We have here a 
definite mechanical result of chemical union, which, if the 
theory of an aether be true, is as certain as any conclusion of 
mathematics, and which would hardly be rendered more certain 
if the physical vision were so sharpened as to be able to see the 
oscillating atom and the medium in which it swings. I write 
thus definitely lest it should be imagined that we are dealing in 
vague conjectures. The connexion of chemical and mechani¬ 
cal phenomena here established must, I think, be pregnant of 
results. 

Further, if, as all the facts declare, radiation and absorption 
are complementary acts, a giving and taking of motion, united 
by a bond of strict proportionality, then it may be affirmed that 
no coincidence in period between the vibrations of a radiating 
body and those of oxygen, hydrogen, or air could make any one 
of these substances a good absorber. They are physically in¬ 
capacitated from communicating motion, and hence in an equal 
degree incapacitated from accepting motion. The form of the 
atom, therefore, or some other attribute than its period of 
oscillation, must enter into the question of absorption. The 
neutrality of the elementary gases in the foregoing experi¬ 
ments does not arise from the accident that a source of heat 
was chosen whose periods did not synchronize with those of 
the gas; for however both might synchronize, the gas would 
still be a bad absorber. Even when the motion of heat which 


BY GASEOUS MATTER. 


99 


their own absorbent power does not enable them to take np is 
mechanically imparted to the atoms, or is communicated to them 
by contact, elementary bodies expend it but sparingly upon the 
luminiferous aether, which accepts all vibrations alike.* 


§ 16 . 

Action of Odours upon Radiant Heat. 

Scents and effluvia generally have long excited the attention 
of observant men. They have formed favourite illustrations 
of the divisibility of matter. Several chapters in the works of 
the celebrated Robert Boyle are devoted to this subject, and 
philosophers in all countries have speculated more or less upon 
the extraordinary tenuity of the matter which is competent to 
produce sensible effects upon the olfactory nerves. We have 
here, of course, materials for a wide inquiry,.which it is quite 
out of my power to undertake at present. I think, however, 
that the apparatus thus far made use of enables us to deal 
with the question in a manner hitherto unattainable. 

The leaves and flowers of a number of dry aromatic plants,f 
obtained from Covent Garden, were stuffed into glass tubes 18 
inches long and a quarter of an inch in diameter. A current 
of dry air was first sent through the tubes for some minutes. 
They were then connected with the exhausted experimental 
tube, with its sources of heat arranged as already described. 
Dry air was then passed over the scented herbs until the 
experimental tube was filled. The consequent deflection was 
noted, and from it the absorbent action of the odorous sub¬ 
stance was deduced. 

The - odour of thyme thus treated intercepted thirty-three 
times the quantity of heat stepped by the air in which it was 
diffused. 


* I can hardly imagine the hands in the spectra of metallic compounds to be 
produced by the vibration of the compound atom. All my experiments show the 
vast influence of chemical union on the rate of oscillation; the metal itself and the 
compound of that metal could hardly, in my opinion, oscillate alike. Hence, the fact 
that the lines, say, of sodium burnt in air, or vaporized by the electric spark, are the 
same as those of chloride of sodium, proves, in my opinion, that decomposition has 
occurred when the bright and constant spectral bands are seeu. 

t I mean * dry ’ in the common acceptation of the term. They were not green, 
but withered ; doubtless, strictly speaking, they contained aqueous vapour. 


) 


> > > 


100 


THE ABSORPTION AND RADIATION OF HEAT 


Peppermint intercepted thirty-four times that quantity. 

Spearmint intercepted thirty-eight times the same amount. 

Lavender produced thirty-two times the action of the air. 

Wormwood forty-one times the action of the air. 

A number of perfumes, obtained from Mr. Atkinson, of Bond 
Street, were examined in the following manner. Small squares 
of dried bibulous paper, all of the same size, were rolled into 
cylinders about 2 inches in length; each of these was moistened 
by an aromatic oil, and introduced into a glass tube between 
the drying-apparatus and the experimental tube. The latter 
being first exhausted, was afterwards filled by a current of dry 
air which had passed over the scented paper. Calling the 
action of the air which formed the vehicle of the perfumes 1, 
the following absorptions were observed in the respective 
cases:— 



Table YII. 


• 

Name of Perfume 

Absorption 
per 100 

Name of Perfume 

Absorption 
per 100 

Pachouli 

. 30 

Lavender . 

. 60 

Sandal Wood . 

. 32 

Lemon 

. 65 

Geranium 

. 33 

Portugal . 

. 67 

Oil of Cloves . 

. 33-5 

Thyme 

. . 68 

Otto of Roses . 

. 36-5 

Rosemary . 

. 74 

Bergamot 

. 44 

Oil of Laurel 

. 80 

Neroli 

. 47 

Cassia 

. 109 


In comparison with the air which carried them info the tube, 
the weight of these odours must be almost infinitely small. Still 
we find that the least energetic in the list produces thirty times 
the effect of the air, while the most energetic produces 109 times 
the same effect. Would it be absurd to entertain the notion 
that, as regards the absorption of radiant heat, the perfume of 
a flower-bed may be more efficacious than the entire oxygen and 
nitrogen of the atmosphere above it ? 

After each scent had been introduced, a stream of dry air was 
admitted at one end of the tube, while the pump was worked 
in connexion with the other. The perfume was thus cleared 
out until the needle returned to 0°. This was often a long 
operation, the odours clung with such tenacity to the ap¬ 
paratus. Even after the zero point had been attained in the case 
of a strong perfume, a few minutes’ rest of the pump sufficed to 
bring the scent from its hiding-places in the crevices and cocks 
of the apparatus, and almost to restore the original deflection. 





BY GASEOUS MATTER. 


101 


The quantity of those residues must be left to the imagination 
to conceive. If they were multiplied by billions they probably 
would not reach the density of ordinary air. 

Fearing that the more active perfumes might possibly preju¬ 
dice the deportment of the more feeble ones which succeeded 
them, I made a series of experiments with the following 
essences, and obtained these results:— 


Camomile Flowers 

Spikenard 

Aniseed. 


Absorption per 100 

. 87 

. 355 
. 372 


Immediately afterwards the experiment with bergamot was 
repeated, and its action found to be exactly the same as that 
recorded in the table. 

In experiments on musk different results were obtained at 
different times. On the 16tli of October some fresh musk 
from the perfumer’s, placed in a small glass tube, had dry air 
carried over it into the experimental tube. The first experiment 
gave an absorption of 74 per cent., the air which carried the 
perfume being unity. A second experiment, in which the air 
was admitted more quickly, gave the absorption 72 per cent. 

It would be idle to speculate upon the quantity of matter 
which produced this result. The stories regarding the un¬ 
wasting character of this substance are well known; suffice it 
to say that a quantity of its odour carried into the tube by a 
current of air of a minute’s duration, produced an effect seventy- 
two times that of the air which carried it. Long-continued 
pumping failed to cleanse the tube and passages of the musk. 
It cannot be volatile, for an amount of ether-vapour which 
produces a far greater action is speedily cleared away, while the 
cocks and connecting pieces of the air-pump had to be boiled 
in a solution of soda before they were fit for use after the expe¬ 
riments with this substance. 

Two perfectly concurrent experiments with ordinary cinna¬ 
mon, in which fragments of the substance were placed in a tube 
and had dry air passed over them, gave an absorption of 53. 

Several kinds of tea, treated in the same manner, produced 
absorptions which varied between 20 and 28 per cent. 

In the teas, cinnamon, musk, and the odorous plants already 
referred to, dry air had been passed over them for some time 
before they were examined. Still a small amount of aqueous 


102 


THE ABSORPTION AND RADIATION OF HEAT 


vapour may have entered with the odours, and thus rendered 
the results to some extent of a mixed character. 

§17. 

Action of Ozone upon Radiant Heat. 

In my last memoir the action of ozone was briefly alluded 
to. The experiments were executed with a brass tu^,e polished 
within, and I was desirous of repeating them with a tube which 
could not be attacked by this extraordinary substance. Expe¬ 
riments with the glass tube, performed on the 16th, 17th, and 
18th of last July, satisfied me that the power of ozone as an 
absorber of radiant heat had not been over-estimated. 

For the purpose of lessening the resistance to the passage of 
the current through the decomposing liquid, large electrodes 
were used in the first experiments. The oxygen thus obtained 
differed but little from ordinary oxygen. 

For my recent experiments I had three decomposing-vessels 
constructed: the first (Ho. 1) had platinum plates of about four 
square inches of surface, which were rolled up to economize 
space; the plates of the second (Ho. 2) had two square inches 
of surface; while those of the third (Ho. 3) had onty a square 
inch of surface each. Humerous experiments with these cells 
gave the following constant results :— 

Electrolytic Oxygen. * 

From plates Absorption per 100 

No. 1.20 

No. 2.34 

No. 3 • >•••• 47 

The absorption by ordinary oxygen being unity. 

A series of experiments executed on the following day gave 
these results:— 

No. 1 ...... 21 

No. 2.36 

No. 3.47 

Here the influence of the size of the electrodes is unmistakable. 

A portion of the plates of Ho. 2 was then cut away so as to 
make them smaller than those of Ho. 3. The oxygen obtained 
with these plates gave an absorption of 


65, 


BY GASEOUS MATTER. 


103 


thus exceeding No. 3 considerably. The plates of No. 3 were 
now reduced so as to make them the smallest of all; the 
oxygen which they delivered gave an absorption of 

85 . 

Fearing the development of heat with these smallest plates, and 
knowing heat to be very destructive of ozone, I surrounded the 
apparatus by a mixture of pounded ice and salt. The absorp¬ 
tion rose immediately to 

136 . 

Had we not been prepared, by the results already recorded, 
for the effect of minute quantities of matter on radiant heat, 5 ve 
could not fail to be struck with astonishment on finding- a 
quantity of ozone, which would elude all attempts on the part 
of the chemist to determine its amount, producing an effect so 
stupendous in comparison with that of common oxygen. I have, 
moreover, strong reason to believe that the effect of the ozone 
is here understated. 

§ 18. 

Experiments of De la Rive and Meidinger. 

All the results here recorded had been for some time obtained, 
when, turning to De la Five’s excellent treatise on electricity, 
I there found the experiments of M. Meidinger on ozone 
referred to. I had never previously heard any allusion made 
to this investigation, and was gratified to find it the record of 
a very interesting piece of work. 

M. Meidinger commences by showing the absence of agree¬ 
ment between, theory and experiment in the decomposition of 
water, the difference showing itself very decidedly in a deficiency 
of oxygen when the current was strong. On heating his electro¬ 
lyte, he found that this difference disappeared, the proper 
quantity of oxygen being always liberated. He at once sur¬ 
mised that the defect of oxygen might be due to the formation 
of ozone; but in what way u 7 as still to be determined. If it 
were due to the greater density of ozone in the tube which 
received the oxygen, the destruction of this substance by heat 
would restore the true volume. Strong heating, however, 
which destroyed the ozone, produced no alteration off-volume. 
Hence M. Meidinger concluded that the observed defect was 
not due to .the ozone mixed with the oxygen itself. He finally 


104 


THE ABSORPTION AND RADIATION OF HEAT 


concluded, and justified liis conclusion by satisfactory experi¬ 
ments, that, the loss of oxygen was caused by the formation of 
peroxide of hydrogen, which being dissolved in the liquid was 
withdrawn from the electrolytic gas. He was further led 
to experiment with electrodes of different sizes, and found the 
loss of oxygen to be more considerable with a small elec¬ 
trode than with a large one; whence he inferred that the 
formation of ozone was facilitated by augmenting the density of 
the current at the place where the electrode and electrolyte meet. 
Nothing could be more different than the methods independently 
pursued by M. Meidinger and me in arriving at the same 
conclusion; and though no doubt of the accuracy of my 
experiments existed in my mind, it was pleasant to find them 
supported in such a remarkable and unexpected way. Since 
the perusal of M. Meidinger’s paper I have repeated his experi¬ 
ments with the decomposition-cells above described, and have 
found that those which yielded the greatest absorption also 
show the greatest deficiency in the amount of oxygen liberated.* 

§19. 

On the Constitution of Ozone. 

The quantities of ozone brought to bear in the foregoing 
experiments must be perfectly unmeasurable by ordinary means. 
No elementary gas that I have examined behaves at all like 
ozone. Its action is like that of olefiant gas, or boracic-ether 
vapour; bulk for bulk it might indeed transcend either. If it 
be oxygen, it must be oxygen packed into groups of atoms, which 
encounter vast resistance in moving through the aether. Two views 
of its constitution are entertained; the one regarding it as a 
form of oxygen, the other as a compound of hydrogen. I 
sought to decide the question in the following way:—Heat 
destroys ozone. If it were oxygen only, heat would convert it 
into the common gas; if it were the hydrogen compound, heat 
would convert it into oxygen plus aqueous vapour. The gas 
alone admitted into the experimental tube would give the 
neutral action of oxygen, but the gas plus aqueous vapour 
would give a sensibly greater action. The dry electrolytic gas 

* I have recently learned that M. de la Rive himself was the first to observe the 
influence of the size of the electrodes on the development of ozone. 


BY GASEOUS MATTER. 


105 


was first caused to pass through a glass tube heated to redness, 
and thence directly into the experimental tube. The experiment 
was repeated with a drying apparatus introduced between the 
heated tube and the experimental tube. The result is, that 
hitherto I have not been able to establish with certainty a 
difference between the two cases. If, therefore, the act of 
heating developed aqueous vapour, I can only say that the most 
powerful experimental tests fail to prove its presence. For 
the present, therefore, I hold that ozone is produced by the 
packing of the atoms of elementary oxygen into oscillating groups — 
that heat dissolves the bond of union, and allows the atoms to 
swing singly, thus disqualifying them for either intercepting or 
generating the motion which in combination they are competent 
to intercept and generate. 


§ 20 . 

Action of Aqueous Vapour upon Radiant Heat.—Experiments 

of Magnus. 

Since these researches were commenced, an eminent experi¬ 
menter has been led by his own inquiries in another field to 
enter upon the investigation of gaseous diathermancy. On the 
7th of February of the present year (1861), Professor Magnus 
communicated to the Academy of Sciences in Berlin a memoir 
‘ On the Transmission of Heat through Gases.’ * The published 
notices of my experiments, commencing in May 1859, had 
escaped his attention, and his work is therefore to be regarded 
as independent of mine. Considering the very different methods 
which we have pursued, the general agreement between us must 
be regarded as remarkable. 

The starting-point of Professor Magnus’s investigation was 
the interesting experiment of Mr. Grove, in which a platinum 
wire heated to whiteness by an electric current is suddenly 
cooled when plunged into hydrogen. This action, which we 
have hitherto been disposed to attribute to the mobility of 
hydrogen, and its consequent high convective power, Professor 
Magnus holds to be an effect of conduction; and this belief 
induced him to examine the conductibility of gases generally. 

* Poggendorff’s Annalen, reprinted in Philosophical Magazine, S. 4. vol. xxii. p. 85. 


106 


TIIE ABSORPTION AND RADIATION OF HEAT 


Tlie mode of experiment "which he adopted led him, not, in my 
opinion, to the establishment of gaseous conductivity at all, but 
to results substantially the same as some of those that I had 
previously obtained. In fact the very experiments devised to 
show conductivity proved in a very striking manner the exist¬ 
ence of atliermancy, or opacity to radiant heat, in the case of a 
considerable number of gases. 

The apparatus of Professor Magnus consisted of two glass 
vessels, one much larger than the other, with their bottoms 
fused together. The larger one being turned upside down, the 
smaller one stood upright on the top of It. The mouth of the 
larger vessel was ground dow T n, so that it could be placed like 
an ordinary receiver on the plate of an air-pump and exhausted, 
while through proper cocks different gases could be afterwards 
admitted into it. 

To the plate of the air-pump on which the vessel was placed 
was attached a thermo-electric pile with wires leading from 
it, through the plate, to a galvanometer; the axis of the pile 
was vertical, one face of it being turned downwards, and the 
opposite face turned upwards towards the common surface of 
the two vessels which had been fused together. 

Water was placed in the uppermost vessel, and caused to 
boil by conducting hot steam through it. Its bottom, which 
formed the top of the lower vessel, was thus heated to a 
temperature of 100° C., and it formed the source of heat made 
use of in the experiments. 

Here, therefore, Professor Magnus had a radiating surface of 
glass—a good radiator—kept at a constant temperature by the 
hot water above it. At a distance from this surface, and turned 
towards it, was the thermo-electric pile, defended from the 
radiation of the surface, or exposed to it, at pleasure, by the 
action of a moveable screen. The entire space between the pile 
and the radiating surface could either be rendered a vacuum, 
offering no resistance to the passage of the calorific rays, or it 
could be filled by a gas the diathermancy of which was to be 
examined. 

The concurrence of the experiments made with this apparatus 
and some of those previously made with mine is, as I have 
stated, remarkable. Some differences, however, exist between 
my friend and myself, a few remarks on which will not be with- 


BY GASEOUS MATTER. 


107 


out tlieir use to those who may afterwards enter upon this 
extensive field of inquiry. 

Experimenting in the ordinary way with his thermo-electric 
pile—that is to say, using one of its faces only—Professor Magnus 
finds that air and oxygen respectively intercept more than 11 
per cent, of the heat emanating from his source of heat, while 
hydrogen cuts off more than 14 per cent. I, on the contrary, 
with the most powerful and delicate means I could employ, failed 
to establish, by experiments made in the ordinary manner, any 
action whatever on the part of these gases.* In fact it was their 
neutrality that drove me to devise the principle of compensation, 
described in the last memoir and briefly referred to at the com¬ 
mencement of this one. I was so particular in the experiments 
which led to the above negative result, that if the absorption 
amounted to one-tenth of that found by Professor Magnus it 
could not have escaped me. Nor is it likely that, if such an 
action existed, Melloni could have concluded that the absorption 
of a column of air fifteen times the length of that employed 
by Professor Magnus was absolutely insensible. 

In the account of experiments published in Memoir I., where 
the source of heat was also 100° 0., and the powerful method of 
compensation was employed, the absorption of air, oxygen, 
and hydrogen is set down at about 0*33 per cent., which is for 
air and oxygen thirty times, and for hydrogen over forty times 
less than that found by Professor Magnus. 

In fixing the above figure for the absorption of these 
gases, I protected myself by assigning the superior limit of 
the effect, but I was morally certain at the time that by the 
improvement of the apparatus in power and delicacy, the effect 
would be made less. In the present inquiry, accordingly, the 
absorption was found to be under 0*1 per cent., which in the 
case of oxygen is less than -Y^-th, and in the case of hydrogen 
less than T ^oth of the effect obtained by Professor Magnus with 
a tube less than half the length of mine. Making every allow¬ 
ance for the difference between our two sources of heat, 
the discrepancy between us ^ still enormous. In fact my con¬ 
clusion is that these gases are practical vacua to radiant heat, 
and that the mixture of oxygen and nitrogen which constitutes 
the body of our atmosphere is the same. 

* Page 12. 




108 


THE ABSORPTION AND RADIATION OF HEAT 


While, however, in the case of the elementary gases the dis¬ 
crepancy between Professor Magnus and myself consists in a 
defect on my part, or in an excess on his, with the powerful com¬ 
pound gases I obtain a considerably stronger action than he does. 
Thus with olefiant gas his absorption amounts to less than 54 
per cent., whereas in mine it amounts to more than 72. This 
last result, however, is only what might be expected, inasmuch 
as the length of gas traversed by the radiant heat was in the 
one case a little under 15 inches, and in the other 33. 

Professor Magnus has further published an account of experi¬ 
ments in which his source of heat was a powerful gas-flame, 
surrounded by a glass cylinder, and provided with a polished 
parabolic mirror to reflect and concentrate the rays. In this 
case the gases were enclosed in a glass tube 1 metre long and 35 
millimetres in diameter, the two ends of which were stopped 
with plates of glass 4 millimetres thick. 

Two series of experiments were executed with this tube, in 
one of which the interior surface was covered with black paper, 
and in the other uncovered. The former method had been 
previously pursued by Dr. Franz; and the result obtained by 
Professor Magnus in the case of atmospheric air and oxygen 
closely agrees with that obtained by Dr. Franz for the same 
gases. Professor Magnus makes the absorption in the case of 
the blackened tube about 2J, and Dr. Franz about 3 per cent., 
for air and oxygen. 

In the case of the unblackened tube, however, the absorption 
was found to be much more considerable. Here air and oxygen 
quenched each 14*75 per cent., while hydrogen intercepted 
16*23 per cent, of the total radiation. This great difference 
between the unblackened and the blackened tube is ascribed 
by Professor Magnus to a change of quality on the part of 
the heat, produced by its reflexion at the interior glass 
surface. 

One of my motives for introducing a glass tube into the pre¬ 
sent inquiry was to enable myself to investigate the question 
raised by this surmise of Professor Magnus. I have failed, 
however, to obtain his result. My naked glass tube, which is 
nearly of the same length as his, gives me an‘action which 
is more than 140 times less than his in the case of air and 
oxygen, and more than 160 times less than his in the case 


BY GASEOUS MATTER. 


109 


of hydrogen. Onr sources of lieat are, it is true, different, hut 
the disadvantage is on my side ; for assuredly the rays from 
a gas-jet are less affected bj the transparent elementary gases 
than those from an obscure source. Were the time at my dis¬ 
posal, I would repeat the experiments with a flame; but this, I 
regret to say, is out of my power at present. 

Another difference between Professor Magnus and m} f self has 
reference to the influence of aqueous vapour. With both the 
gas-flame and the boiling water as sources of heat, he finds the 
effect of dry air to be precisely the same as that of air which he 
has allowed to pass in minute bubbles through water, and thus 
saturated with aqueous vapour. 

I was engaged in experiments on this substance when my 
other duties compelled me to close this inquiry for a time. It 
may, however, be safely affirmed that not only is the action of 
aqueous vapour on radiant heat measurable, but that this action 
may be made use of as a measure of atmospheric moisture , the tube 
used in my experiments being thus converted into a hygrometer of 
surpassing delicacy. Unhappily, as in other cases touched upon 
in this memoir, I have been unable to give this subject the 
desired development; but the results obtained are neverthe¬ 
less interesting. 

On a great number of occasions the air sent directly from 
the laboratory into the experimental tube was compared with 
the same air after it had been passed through a drying-appa¬ 
ratus. Calling the action of the dry air unity, or supposing it 
rather to oscillate about unity (for the temperature of the 
source of heat varied a little from day to day), on the following 
days the annexed absorptions were observed with the undried 
air of the laboratory :— 

Absorptions by undried air. 


October 23rd 

. 63 

November 1st . 

• 

. 50 

October 24th 

. 62 

November 4th . 

• 

. 58 

October 29th 

. 65 

November 8th . 

• 

. 49 

October 31st 

. 56 

November 12th . 

• 

. 62 


Nearly T %ths of the above effects are due to aqueous vapour; 
which, therefore, in some instances exerted nearly sixty times the 
action of the air in which it was diffused. 

The experiments executed on aqueous vapour have been very 



110 


THE ABSORPTION AND RADIATION OF HEAT 


numerous and varied. Differing, as I did, from so cautious and 
able an experimenter as Professor Magnus, I spared no pains to 
secure myself against error. Air moistened in various ways, 
sometimes by allowing small bubbles of it to ascend through 
water, sometimes dividing it by sending it through the pores 
of common cane immersed in water, has been experimented 
with. Between the drying apparatus and the experimental 
tube tubes have been introduced containing fragments of 
glass moistened with water, and the air allowed to pass 
over them; in all such cases large effects were obtained, the 
absorption being usually more than eighty times that of dried 
air. Fragments of unwetted glass, which had been merely ex¬ 
posed to the air of the laboratory, had dry air led over them 
into the experimental tube ; the absorption was fifteen times 
that of dried air.* A roll of bibulous paper, taken from one 
of the drawers of the laboratory, and to all ajDpearance per¬ 
fectly dry, was enclosed in a glass tube, and dry air carried 
between its leaves. The experiment was made five times in 


succession with the same 
were observed:— 

paper, and the following absorptions 

Absorption per 100 

No. 1 

. 72 

No. 2 

.62 

No. 3 

.62 

No. 4 

.47 

No. 5 

.47 


In fact, the action of aqueous vapour is exactly such as might 
be expected from the vapour of a liquid which Melloni found to 
be the most powerful absorber of radiant heat of all that he 
had examined. 


§ 21 . 

Night-Moisture on the Interior Surface of Experimental Tube. 
—Abandonment of Rock-salt Plates. 

Every morning, on commencing my experiments, I had an 
interesting example of the power of glass to gather a film of 
aqueous moisture on its surface. The air of the laboratory being 
removed from the experimental tube, on allowing dry air to enter 
for the first time, the needle would move from 0° to 50°. On 

* These experiments have a direct bearing on the subsequent ones of Professor 
Magnus. [1872.] 




BY GASEOUS MATTER. 


Ill 


pumping out it would return to 0°, and on letting in dry air a 
second time it would swing almost to 40°. Repeated exhaus¬ 
tions caused this action to sink almost to nothing. These results 
were entirely due to the vapour collected during the night in an 
invisible him on the inner surface of the tube, which was removed 
by the air on entering, and diffused through the tube. When 
the dry air entered at the end of the tube nearest the source 
of heat, on the first and second admissions, and sometimes even 
on a third, the vapour carried from the warm end to the cold 
end was precipitated as a mist upon the surface of the glass 
for a distance sometimes of nearly a foot. The mistiness 
always disappeared on pumping out. It is needless to remark 
that facts of this character, of which many could be cited, were 
not calculated to promote incautiousness on my part. I saw 
very clearly how easy it was to fall into the gravest errors, and 
took due precautions to prevent myself from doing so. 

Knowing that a solution of salt was almost as opaque to ra¬ 
diant heat as water itself, I was careful to examine whether the 
effects observed with aqueous vapour might not be due to the 
precipitation of the vapour on the rock-salt surfaces. The sub¬ 
stance is well known to be very hygroscopic ; and during the 
last three years the knowledge of this fact has rendered me 
careful to remove the polished plates every evening from the 
apparatus, and to keep them in perfectly dry air. Still, when it 
is remembered that the air on entering the tube is raised in 
temperature and thus enabled to maintain a greater amount 
of vapour, and that the tube and plates of rock-salt form the 
channel for a flux of heat from the radiating source, the likeli¬ 
hood of precipitation occurring will seem but small. On ex¬ 
amining the plates, moreover, after the undried air of the 
laboratory had been experimented with, no trace of precipitated 
moisture was observed upon their surfaces. 

But, to place the matter beyond all doubt, I abolished the 
plates of rock-salt altogether, and operated thus An india- 
rubber bag B (Fig. 9) was filled with air, and to its nozzle a 
T-piece, with the cocks Q Q', was attached. The cock Q' was con¬ 
nected with two tubes, TJ' IT, each of which was filled with frag¬ 
ments of glass moistened with distilled water. The cock Q w as 
connected with the tubes U U, each of which was filled with frag¬ 
ments of glass moistened by sulphuric acid. The otliei ends of 






Fig. 9. 


112 


THE ABSORPTION AND RADIATION OF HEAT 


these two series of tubes were connected with the cocks 0 O'; and 
from the T-jhece between these cocks a tube led to the end E' of 
the open experimental tube T. The cock A at the other end 



of the experimental tube was placed in connexion with an air- 
pump. The pile P, the screen S, and the compensating cube O' 
were used as in the other experiments. E is the end of the front 

















































































































































































BY GASEOUS MATTER. 


113 


chamber, and C the source of heat. In some experiments I had 
the end E closed by a plate of rock-salt, in others it was allowed 
to remain open, a distance of about 12 inches intervening' 
between the radiating surface and the open end E' of the expe¬ 
rimental tube. 

Closing the cocks Q and 0, and opening Q' and O', gentle 
pressure being applied to the bag B, a current of moist air was 
slowly discharged at the end E' of the experimental tube. The 
pump in connexion with A was then worked, and thus by degrees 
the air was sucked into the tube T. The deflection of the galva¬ 
nometer was 30°, when the moist air filled the tube as completely 
as the arrangement permitted,*—this deflection being due to 
the predominance of the compensating cube over the radiating 
source C. 

The cocks Q' and O' were now closed, and Q and 0 opened; 
proceeding as before, a current of dry air was discharged at E', 
and this air was drawn into the tube T in. the manner just 
described. The moist air was thus displaced by dry; and, 
while the displacement was going on, the galvanometer was 
observed through the distant telescope. The needle soon 
began to sink, and slowly went down to zero, proving that a 
greater quantity of heat passed through the dry than through 
the moist air. The wet air was substituted for the dry, and the 
dry for the wet twenty times in succession, with the same con¬ 
stant result: the entrance of the humid air caused the needle 
to move from 0° to 30°, while the entrance of dry air caused it 
to fall from'30° to 0°. The air-pump was resorted to, because 
I found in attempting to displace the air by the direct force of 
the current from B, the temperature of the pile, or of the source 
of heat, was so affected by the fresh air as to confuse the result. 
I may remark that not only have I operated thus for days with 
aqueous vapour, but every result obtained with vapours generally 
lias been thus confirmed , so that all doubt as to the applicability 
of the rock-salt plates to researches of this nature may, I 
think, be abandoned.! 

* Still, of course, only partially. 

t This proved to be source of error in Professor Magnus’s subsequent experiments. 
See Memoir IV. It is sheer want of time that prevents me from describing more 
particularly the numerous experiments executed with open tubes. 


8 



114 


THE ABSORPTION AND RADIATION OF HEAT 



Proposed Solution of Discrepancies. 


Whence, then, arise those differences between Professor 
Magnus and myself? I am quite convinced that his experi¬ 
ments have been made with the utmost care which it is possible 
to bestow upon scientific work, and the differences between us 
are, in my opinion, to be referred to a radical defect in his 
apparatus. His desire to do away with plates of all kinds 
between his source of heat and his pile, caused him to bring 
his gas into direct contact with his source of heat. I was on the 
point of falling into the same error; but a series of experiments 
executed with reference to this point, so early as July 26, 1859, 
proved the accuracy of the results to be entirely compromised 
by bringing the gas to be examined into contact with the 
source of heat. In one experiment where this occurred I ob¬ 
tained an action forty times what I knew it ought to be, being 
thereby confirmed in my opinion as to the necessity of inter¬ 
posing a vacuous chamber in front of the experimental tube. 
Let me here record a few experiments made on the 4th of last 
November in connexion with this subject. 

Having first made sure that the drying apparatus was in 
perfect condition—the air of the laboratory producing, when 
sent through it, an absorption of 1—this same dry air was sent 
into the front chamber, that is, into direct contact with the 
source. The galvanometer needle moved as it does in the case 
of absorbent gases, and at the end of two minutes it declared a 
loss of heat equivalent to an absorption of 50. The front 
chamber is 8 inches in length ; the experimental tube 83 inches; 
hence a column of 8 inches, in contact with the radiating sur¬ 
face, produced at least fifty times the effect of a column more 
than four times as long when the air was separated from the 
radiating surface. 

The foregoing experiment was made three times in succession, 
and after two minutes * the needle was found pointing to precisely 
the same degree; the lowering of the source of heat was per¬ 
fectly constant and regular, and in all cases showed a loss 
equivalent to an absorption of 50. 

It will be remembered that Professor Magnus obtained a 
* The time of exposure of Professor Magnus’s pile.' 


BY GASEOUS MATTER. 


115 













greater absorption with hydrogen than with either oxygen or 
air. This result is perfectly explained by reference to the 
quicker convection of this gas. I operated with hydrogen as 
with air, first satisfying myself that a column of the gas 33 
inches long exercised an absorption less than unity: in fact it 
could not be measured. The same hydrogen introduced into 
the front chamber, and allowed to remain there for two minutes, 
caused a withdrawal of heat equivalent to an absorption of 65. 
Now the action of air in Professor Magnus’s experiments is to 
that of hydrogen as 

11-12 : 14-21, 

or as 

50 : 64, 

while my results of convection are as 

50 : 65. 

The coincidence is so perfect that one is disposed to regard it 
as in part accidental.* 

Substantially the same remarks apply to the experiments 
with the glass tube stopped with plates of glass 4 millimetres 
thick. According to Melloni, 61 per cent, of the rays of a Loca- 
telli lamp are absorbed by a plate of glass only 2*6 millimetres 
thick. True, Professor Magnus surrounded his flame by a glass 
cylinder; and this, it may be urged, partially sifted the heat of 
the lamp before it reached the end of the tube. But in so 
doing the glass cylinder itself must become intensely heated; 
and to the heat of the cylinder the glass ends of the tube would 
be opaque ; they would absorb it all. Cold air admitted into 
such a tube is exactly similar to cold air let into my front 
chamber; it chills the secondary sources of heat, and main¬ 
tains that chill by convection. The heat applied may, in fact, 
be thus analysed:—1. We have a portion of the heat from 
the lamp passing without losing the radiant form through the 
tube direct to the pile; 2, a portion of that flux arrested by the 
first glass plate ; 3, a smaller portion arrested by the second 
glass plate; 4, the heat radiated by the first glass plate towards 
the second, and wholly absorbed by the latter; 5, the heat 
radiated by this latter against the pile. This analysis enables 
us to clearly understand how Professor Magnus obtained an 

* I do not think it accidental. This result is entirely in accordance with those of 
Count Rumford. [1872]. 






116 THE ABSORPTION AND RADIATION OF HEAT 

absorption of only 2J per cent, with the tube blackened 
within, and as much as 14-75 per cent, with the unblackened 
one. With the unblackened tube, both the source of heat and 
the plate of glass nearest to it sent a copious flux down the 
tube to the plate at the opposite end ; for here the oblique rays 
are in great part reflected by the interior surface. With the 
blackened tube this oblique radiation is cut off, the rays incident 
on the interior surface being absorbed. Hence the plate of glass 
adjacent to the pile must be much more intensely heated with 
the unblackened tube than with the blackened one. The 
difference in the amount of heat impinging on the pile-end 
plate in the respective cases is rendered very manifest by the 
experiments of Professor Magnus himself; who finds the heat 
transmitted by the uncoated tube to be twenty-six times that 
transmitted by the coated one. What, therefore. Professor 
Magnus ascribes to a change of quality by reflexion, is per¬ 
fectly explained by reference to the greater heating, and conse¬ 
quent greater chilling by the cold air, of the plate of glass close 

to tlie pile. 

The difference between Professor Magnus and myself as 
regards the action of aqueous vapour admits also of easy expla¬ 
nation. His effect being one of convection, and not of absorption, 
the quantity of vapour present in his experiments—probably 
not more than 1 per cent, of the volume of the gas, certainly 
not 2 per cent.—vanished as a convecting agent, in comparison 

with the air. 

It is hardly necessary to repeat these reflexions with reference 
to the experiments of Hr. Franz. The mistaking of the chilling 
of his plates for absorption caused him to find no difference of 
effect when he doubled the length of his tube. With a tube 
450 millimetres long, he found precisely the same absorption as 
with one of 900. He also found the action of carbonic acid to 
be the same as that of air, although at atmospheric pressures 
the action of the former is 90 times that of the latter* He 
found the vapour of bromine more destructive to radiant heat 

* The sensible equality of all the transparent gases and air was regarded as self- 
evident by Dr. Franz. ‘ It might be seen,’ he writes, ‘ from the outset that no decided 
difference would be observed between them ’ (p. 342). Similarly, Professor Magnus, 
speaking of aqueous vapour, writes, ‘ Although it might be foreseen with certainty 
that the small amount of aqueous vapour in the air could have no influence on the 
radiation,’ &c. (p. 43). 


BY GASEOUS MATTER. 


117 


than nitrous acid gas, whereas the latter is beyond comparison 
the most destructive. The heat rendered latent by the evapora¬ 
tion of the bromine of course augmented the chill of his plates, 
and thus magnified the effect which in reality he was measuring. 

§ 23. 

Action of Atmospheric Envelope.—Possible Experimental Deter- 
mination of the Temperature of Space. 

As a dam built across a river causes a local deepening of the 
stream, so our atmosphere, thrown as a barrier across the terres¬ 
trial rays, produces a local heightening of the temperature at 
the earth; s surface. This, of course, does not imply indefinite 
accumulation, any more than the river dam does, the quantity 
lost by terrestrial radiation being, finally, equal to the quantity 
received from the sun. The chief intercepting substance is the 
aqueous vapour of the atmosphere,* the oxygen and nitrogen of 
which the great mass of the atmosphere is composed being 
sensibly transparent to the calorific rays. Were the atmosphere 
cleansed of its vapour, the temperature of space would be 
directly open to us ; and could we under present circumstances 
reach an elevation where the amount of that vapour is insensible, 
we might determine the temperature of space by direct experi¬ 
ment. Colonel, now General, Stracliey has written an admir¬ 
able paper on the aqueous vapour of the atmosphere,f in which 
he shows that the amount of vapour diminishes much more 
rapidly with the elevation than might be inferred from Dalton’s 
law. 

It might therefore be possible to reach a height where, by 
preserving one face of a thermo-electric pile at the temperature 
of the locality, the other, protected from all terrestrial radiation, 
and turned to the zenith, would assume the temperature of 
space,J while the consequent galvanometric deflection would 

* The mildness of an island climate must be in part due to this cause. The direct 
tendency of the vapour is to check sudden fluctuations of temperature. Where it is 
absent, as at the surface of the moon, such fluctuations must be enormous. The 
face turned towards the sun drinks in the solar rays without let or hindrance, while 
the radiation of the face turned from the sun pours unchecked into space. 

f Phil. Mag. S. 4. vol. xxiii. p. 152. 

f A well of cold air would be formed within the conical reflector, the lowest stratum 
of the well sharing the temperature of the face of the pile. 


118 


THE ABSORPTION AND RADIATION OF HEAT 


give us the means of determining the difference in tem¬ 
perature between the two faces of the pile. Knowing, there¬ 
fore, the temperature of the locality, we could infer from it the 
temperature of stellar space. Many eminent writers, it is true, 
have supposed the upper atmospheric regions to be colder than 
space, the temperature being lowered by the radiation of the 
aerial particles, just as the temperature of a grass-blade is 
lowered by radiation on a clear night. This notion must, I 
think, be abandoned; for experiment leads us to conclude that 
air, and particularly air in the higher atmospheric regions, 
behaves as a vacuum both as regards radiation and absorption. 

§ 24. 

Remarks on the Experimental Evidence of Gaseous Conduction .— 

Influence of Density on Convection.—Internal Friction of Air . 

In his paper on the conduction of heat by gases, Professor 
Magnus has adduced some striking experiments to show that the 
cooling of an incandescent wire in hydrogen is not due to the 
convection of the gas. He finds that when the wire is enclosed 
in a narrow tube, with only a thin film of the gas surrounding it, 
and where therefore currents, in the ordinary sense, can hardly 
exist, the gas still exercises its cooling power. It had often 
occurred to me to make this experiment; and when intelligence 
of its successful performance by Professor Magnus first reached 
me I adopted his conclusion, that the cooling is due to con¬ 
duction. 

Reflexion, however, caused me to change this opinion. Sup¬ 
pose the wire to be stretched along the axis of a wide cylinder 
containing hydrogen, we should have convection, in the ordinary 
sense, on heating the wire. Where does the heat thus dispersed 
ultimately go ? It is given up to the sides of the cylinder, and 
if we narrow the cylinder we simply hasten the transfer. The 
process of narrowing may continue till a tube like that used by 
Professor Magnus is the result; the convection between centre 
and sides will still continue, and produce the same cooling 
effect as before. Whether we assume conduction or convection on 
the part of the gas, the tube surrounding the wire must possess 
sufficient conducting power to carry the heat off, otherwise it 
would become incandescent itself by the accumulation of the heat. 


BY GASEOUS MATTER. 


110 


The further reasoning of Professor Magnus in connexion with 

O o 

this subject is of extreme ingenuity. He contends that there 
is no reason why stronger currents should establish themselves 
in hydrogen than in other gases. Currents, he urges, are due to 
differences of density produced by the expansion of a portion of 
the gas by heat. But hydrogen actually expands less than other 
gases, and hence the differential action on which the currents 
depend is less in this gas than in the others. Professor Magnus 
alludes to the friction of the particles against each other, but 
considers this ineffective. 

This reasoning leads us to the threshold of a question which 
might form the subject of a long and profitable investigation. 
The question is :—For a given difference of density, is not the 
mobility of hydrogen greater than that of the other gases '? The 
experiments recorded in § 22, where different gases were brought 
into direct contact with the source of heat, seem to answer this 
question in the affirmative. I have had no time to pursue the 
question regarding hydrogen ; but a few experiments have been 
made which show in a very striking manner the influence of 
density on the mobility of a gas. 

Having first so purified atmospheric air as to render it sen¬ 
sibly neutral to radiant heat, I allowed 15 inches of it to enter 
the front chamber F (see Frontispiece ), and there to come into 
contact with the source of heat. Convection, of course, im¬ 
mediately set in, and its amount was acc.uratel^ measured by 
the quantity of heal withdrawn from the radiating surface ; 
this, expressed in the units adopted throughout this memoir, 
was 62. 

The quantity of gas in the front chamber was then doubled— 
in other words, increased to a whole atmosphere ; the withdrawa 
of heat was expressed by the number 68. 

In the last experiment we had double the number of atoms 
loading themselves with heat and carrying it away; if their 
motion had been as quick as that of the atoms when half an 
atmosphere was used, they would have withdrawn sensibly 
double the amount of heat; but the fact is that half an atmo¬ 
sphere carried off 62, while a whole atmosphere carried off 68 ; 
hence the absolute swiftness of the atoms in the case of the 
denser air must be very much less than in the case of the rarer. 
In fact, the amount of heat withdrawn will be proportional on 



120 THE ABSORPTION AND RADIATION OF HEAT 

tlie one hand to the number of carrying particles, and on the 
other to the velocity with which they move; hence if v and v 
be these velocities, we have 

62 v v 62 
— = —,, or ——. 

68 2(/ 34 

Thus, while the atoms of the rarer gas travel G2 units in a 
second, those of the denser gas travel only 34. 

This retardation can, I think, arise from nothing else than 
the resistance offered by the particles of the air to the motion 
of their fellows. It must be borne in mind that the smallness 
of the increment observed on doubling the amount of gas was 
not due to the partial exhaustion of the source of heat by 
the first half atmosphere of gas. The heat of the source was 
such that the withdrawal of 64 of our units could not sensibly 
affect the subsequent convection. 

Here, then, we see what a powerful effect density, or the in¬ 
ternal resistance which accompanies density, has on the mobility 
of a gas; and there is every reason to suppose that the mobility 
of hydrogen is due to the comparative absence, in its case, of 
internal friction. However this may be, the foregoing experi¬ 
ment enables us to draw some important inferences. 

Local storms at great heights must be greatly facilitated by 
the mobility of the particles of the air. Storms are cases of 
convection on a large scale, and in our front chamber we had 
one in miniature. 

In the summer of 1859 I was fortunate enough to induce 
Professor Franldand to accompany me to the summit of 
Mont Blanc, and to determine the comparative rates of combus¬ 
tion there and in the valley of Chamouni. Six composite 
candles were burnt for an hour at Chamouni, and the loss of 
weight determined. The same candles were lighted for the 
same time on the summit of the mountain, and the consumption 
ao-ain determined. Within the limits of error, the consumption 
above was equal to that below. The light below was immensely 
greater than that above, still the amount of stearine consumed 
in the two cases was sensibly the same. Professor Frankland 
surmised that this might be due to the greater mobility of the 
rarefied air, which allowed a freer interpenetration of the flame 


BY GASEOUS MATTER. 


121 


by the oxygen ; * and tlie foregoing experiments show that the 
augmentation of mobility is just such as would account for the 
observed effect. 

* The influence of interpenetration is well seen in the exposed gas-jets of London, 
particularly in the butchers’ shops on a Saturday night. A gust of wind, which 
carries oxygen to the centre of a flame, suddenly deprives it of light. A simple and 
beautiful experiment consists of passing a lighted candle swiftly to and fro through 
the air ; the white light reduces itself to a pale-blue band. Bunsen’s burner is an 
illustration in the same line. 












































' ’ 












. 




































III. 


ON TIIE RELATION OF RADIANT HEAT TO 

AQUEOUS VAPOUR. 






ANALYSIS OF MEMOIR III. 


In the analysis of Memoir II. the differences which had arisen between 
Professor Magnus and myself regarding the action of dry air on the one hand, and 
of aqueous vapour on the other, are briefly adverted to, and in the concluding 
sections of this memoir the subject is discussed and a solution of the discrep¬ 
ancies is offered. Professor Magnus had previously shown the danger arising 
from the hygroscopic character of rock-salt$ and I, in the memoir referred to, 
replied by definite experiments to this objection. 

In the summer of 1862 he came over to the International Exhibition and I 
had the great pleasure of spending a good deal of time in his genial company. 
This was a favourable opportunity for settling our differences, which were the 
subject of frequent conversation between us. He did me the pleasure to come 
to the Royal Institution to witness my experiments, and it was also his wish 
to show me his arrangements and to test them in my presence. This wish, 
however, his incessant occupations prevented him from carrying out. 

It was first proved to his satisfaction that the method of compensation, re¬ 
garding which I once observed him shake his head in doubt, was capable of 
the last degree of precision. In an experimental tube closed with rock-salt, I 
showed him the neutrality of dry air, and the activity of humid air. While 
the tube remained filled with the latter I removed the rock-salt plates and 
placed them in his hands for inspection. Pie looked at them, passed his finger 
and his dry handkerchief over them, and in the frankest manner exclaimed 
‘ there is no moisture there.’ I then repeated the experiments with an open 
tube, and over and over again, displacing moist air by dry, and dry air by moist, 
showed him by precise and concurrent measurements the constant difference 
subsisting between them. 

I thought it due to him to pay strict attention to every objection he raised, 
whether in his published papers, his letters, or his conversation. He once 
mentioned to me his having found that a layer of air 12 inches deep sufficed to 
absorb all rays that air was capable of absorbing; * and he contended that the 
distance between the end of my experimental tube and my pile, owing to the 
length of its conical reflector, was sufficient to remove most of the heat taken 
up by air. This being the case, the neutrality of dry air followed in my expe¬ 
riments as a natural consequence. For, he rightly and ingeniously contended, 
assuming air to possess the alleged power, the fact that my pile stood beyond 
the experimental tube made no difference ; because the introduction into the 
experimental tube of its charge of dry air merely transferred the absorption 
to a different part of the path traversed by the heat-rays, but did not alter the 

* I understood him to say that he had prepared a paper for Poggendorff, in which this 
and other remarkable results were established. But I have never been able to find the 
paper. 


ANALYSIS OF MEMOIR III. 


125 


amount of the absorption. lie also one day drew my attention to tbe sunbeams 
slanting through the dusty air of London, and remarked good-humouredly, 
‘ There is the source of your absorption.’ My first care in 1862 was to meet 
these objections. 

In the first experiment of the following memoir the reflector of the pile is 
placed within the experimental tube, the face of the pile being only ^th of 
an inch distant from the plate of rock-salt. The distance, it was conceded, 
could produce no sensible absorption. The arrangement is also to be recom¬ 
mended because of the security it ensures agaiust moisture; the heat being 
concentrated upon a small portion of the central area of the plate of salt. 

• The results obtained with this arrangement were precisely the same as the 
former ones. 

To meet the objection regarding London air, I sent special messengers to 
Hyde Park, Primrose Hill, Hampstead Heath, and Epsom Downs, and had air 
from these places. I made an expedition myself to the Isle of Wight, and 
carried home specimens of air from various parts of the island. The experi¬ 
ments made with all these samples of air entirely corroborated the previous 
ones. 

London air, moreover, was purified and dried, until its action on a powerful 
beam of heat was insensible. It was then sent over fragments of glass moistened 
with distilled water. No smoke or .dust could here mix with it; still its 
deportment was in perfect accord with the other experiments. 

Dry smoke, thicker than it is ever seen in London streets, was then purposely 
sent into the experimental tube. Its action was found to be only a fraction of 
that of aqueous vapour. 

I then sought to do away with the experimental tube itself, and to discharge 
dry air and moist alternately between the source of heat and the pile. The 
observed effect was small, but distinct. This experiment, however, which was 
of the most extreme delicacy, I should like to confirm by repetition. 

The question whether condensation occurs on the interior of the experimental 
tube so as to diminish its reflective power is considered. Humid air is admitted 
in varying quantities, and it is found that the absorption of radiant heat is 
accurately proportional to the quantity of vapour present. Such proportionality, 
it is urged, could hardly arise from the supposed condensation. 

The bearing of this property of aqueous vapour upon various problems and 
phenomena of meteorology is then pointed out. The great daily range of the 
thermometer in dry climates; the production of frost at night even in Sahara; 
the cold of the table-land of Asia; the contrast between day and night on 
mountains; the artificial production of ice in India; Leslie’s significant remarks 
on his cethrioscope , where he shows that days of equal atmospheric clearness differ 
widely from each other as regards the power of the air to stop terrestrial radia¬ 
tion. * All these observations are in harmony with, and are indeed explained, 
by this newly discovered property of aqueous vapour. 












, 









































































' 
















" 

















' 









« 

J ; 






































III. 


ON THE RELATION OE RADIANT HEAT TO 

AQUEOUS VAPOUR* 

§ 1 . 

Objections to Rock-salt Plates considered.—New Experimental 

Arrangement. 

I have already given an account of experiments which 
brought to light the remarkable fact that the body of our 
atmosphere—that is to say, the mixture of oxygen and nitrogen 
of which it is composed—is a comparative vacuum to the 
calorific rays, its main absorbent constituent being the aqueous 
vapour which it contains. It is very important that the minds 
of meteorologists should be set at rest on this subject—that 
they should be able to apply, without misgiving, this newly 
revealed physical property of aqueous vapour; for it is certain 
to have numerous and important applications. I therefore 
thought it right to commence my investigations this year with 
a fresh and special series of experiments upon atmospheric 
vapour, which I have now the honour to submit to my 
readers. 

Rock-salt is a hygroscopic substance. If we breathe on a 
polished surface of rock-salt, the affinity of the substance for the 
moisture of the breath causes the latter to spread over it in a 
film which exhibits brilliantly the colours of thin plates. The 
zones of colour shrink and finally disappear as the moisture 
evaporates. Visitors to the International Exhibition of this 
year may have witnessed how moist were the pieces of rock-salt 
exhibited in the Austrian and Hungarian Courts. This property 
of the substance has been referred to by Professor Magnus as a 

* Received by the Royal Society November 20, and read before the Society 
December 18, 1862. Philosophical Transactions, part i. for 1863, and Philosophical 
Magazine for July 1863. 



128 


THE RELATION OF RADIANT HEAT 


possible cause of error in my researches on aqueous vapour; a 
film of brine deposited on the surface of the salt would, he urges, 
produce the effect ascribed to the aqueous vapour. I will, in 
the first place, describe a method of experiment by which even 
an inexperienced operator may avoid all inconvenience of this 
kind. 

In the plate which accompanies my former paper, the thermo¬ 
electric pile is figured with two conical reflectors, both outside 
the experimental tube; in my present experiments the reflector 
which faced the source of heat is placed within the experimental 
tube, its narrow aperture, which usually embraces the pile, 
abutting against the plate of rock-salt which stops the tube. 
Fig. 10 is a sketch of this end of the experimental tube. 

Fig. 10. 



The edge of the inner reflector fits tightly against the interior 
surface of the tube at ah; c d is the diameter of the wide end of 
the outer reflector supposed to be turned towards the ( compen¬ 
sating cube 5 situated towards C'.* The naked face of the pile P 
is turned towards the plate of salt, being separated from the 
latter by an interval of about -^th of an inch. The space 
between the outer surface of the interior reflector and the inner 
surface of the experimental tube is filled with fragments of 
freshly-fused chloride of calcium, intended to keep the circum¬ 
ferential portions of the plate of salt perfectly dry. The flux of 
heat coming from the source C, being converged upon the 
central portion of the salt, completely chases every trace of 
humidity from the surface on which it falls. 

§ 2 . 

Objection to Employment of London Air considered.—Radiation 
through Air from Various Localities. 

With this arrangement I repeated all my former experiments 
on humid and dry air. The result was the same as before. On 

* I here assume an acquaintance with my last two memoirs, in which the method 
of compensation is described. 









TO AQUEOUS VAPOUR. 


129 


a day of average humidity the quantity of vapour diffused in 
London air produced upwards of 60 times the absorption of the 
air itself. 

It had been suggested to me that the air of our laboratory 
might be impure; the suspended carbon particles in a London 
atmosphere had also been mentioned to me as a possible cause 
of the absorption ascribed to aqueous vapour. With regard 
to the first objection, I may say that the same results were 
obtained when the apparatus was removed to a large room 
at a distance from the laboratory; and with regard to the 
second cause of doubt, I met it by procuring air from the 
following places:— 

1. Hyde Park. 

2. Primrose Hill. 

3. Hampstead Heath. 

4. Epsom Pace-Course. 

5. A field near Newport, Isle of Wight. 

6. St. Catharine’s Down, Isle of Wight. 

7. The sea-beach near Black Gang Chine. 

The aqueous vapour of the air from these localities exerted absorp¬ 
tions from 60 to 70 times that of the air in which the vapour was 
diffused. 

I then purposely experimented with smoke, by carrying air 
through a receiver in which ignited brown paper had been 
permitted to smoulder for a time, and drying it afterwards. 
It was easy, of course, in this way to intercept the calorific 
rays; but, adhering to the lengths of air actually experimented 
on, it was proved that, even when the east ivind blows, andjpours 
the carbon of the city upon the West End of London, the heat 
intercepted by the suspended carbon particles is but a minute 
fraction of that absorbed by the aqueous vapour. 

Further, the air of the laboratory was so well purified that its 
absorption was less than unity; the purified air was then con¬ 
ducted through two U-tubes filled with fragments of clean glass 
moistened with distilled water. Its neutrality when dry proved 
that all prejudicial substances had been removed from the air; 
and in passing through the U-tubes it could have contracted 
nothing save the pure vapour of water. The vapour thus carried 
into the experimental tube exerted an absorption 90 times as great 
as that of the air which carried it. 

9 








130 THE RELATION OF RADIANT HEAT 

I have had the pleasure of showing the experiments on atmo¬ 
spheric aqueous vapour to several distinguished m£n, and among 
others to Professor Magnus. After operating with common 
undried air, which showed its usual absorption, and while the 
undried air remained in the experimental tube, 1 removed the 
plates of rock-salt from the tube and submitted them to his 
inspection. They were as dry as polished rock-crystal or 
polished glass; their polish was undimmed by humidity; and 
a dry handkerchief placed over the finger and drawn across the 
plates left no trace behind it.* 

Remark .—I would make one additional remark on the above 
experiments. A reference to the plate which accompanies the 
last two memoirs will show the thermo-electric pile standing, 
with its two conical reflectors, at some little distance from the 
end of the experimental tube. Hence, to reach the pile after it 
had quitted the tube, the heat had to pass through a length of 
air somewhat greater than the depth of the reflector. It has 
been suggested to me that the calorific rays may be entirely 
sifted in this interval—that all rays capable of being absorbed 
by air may be absorbed in the space intervening between the 
experimental tube and the adjacent face of the pile. If this 
were the case, then the filling of the experimental tube itself 
with dry air would produce no sensible absorption. Thus, it 
was imagined, the neutrality of dry air which my experiments 
revealed might be accounted for, and the difference between 
myself and Professor Magnus, who obtained an absorption of 
12 per cent, for dry air, explained. But I think the hypothesis 
is disposed of by the foregoing experiments; for here the 
reflector which separated the pile from the tube no longer 
intervenes, and it cannot be supposed that in an interval of 
_i_th of an inch of air an absorption of 12 per cent, has taken 

* The present number of the Monatsbericht of the Academy of Berlin contains an 
account of some experiments executed with plates of rock-salt by Professor Magnus. 
The plates which stopped the ends of a tube were so far wetted by humid air tha 
the moisture trickled from them in drops. As might be expected, the plates thus 
wetted cut off a large amount of heat. The experiments are quite correct, but they 
have no bearing on my results. In the earlier portions of my journal many similar 
cases are described. In fact, it is by making myself, in the first place, acquainted 
with the anomalies adduced that my results have been rendered secure. I may 
add that the communication just referred to was made to the Academy of Berlin 
before Professor Magnus had an opportunity of examining my rock-salt plates. I do 
not think he would now urge this objection against my mode of experiment. 


TO AQUEOUS VAPOUR. 


131 







place. If, however, a doubt on this point should exist, 
state that I have purposely sent 
radiant heat through an interval 
of 24 inches of dry air previous 
to permitting it to enter the ex¬ 
perimental tube, and found all 
effects to be the same as when 
the beam had traversed 24 inches 
of a vacuum. 


I can 





0 




P 


$ 


fcX) 

ft 





§ 3 . 

Radiation through Open 
Tubes. 

In confirmation of the results 
obtained when the experimental 
tube was stopped by plates of 
rock-salt, the following experi¬ 
ments have been recently made 
with a tube in which no plates 
were used. S (fig. 11) is the 
source of heat, and S T the front, 
chamber which in ordinary expe¬ 
riments is kept exhausted. This 
chamber is now left open. A B is 
the experimental tube, with both 
its ends also open. P is the ther¬ 
mo-electric pile, the anterior face 
of which receives rays from the 
source S, while its posterior sur¬ 
face is warmed by the rays from 
the compensating cube C'. At 
c and d are two stopcocks—that 
at c being connected with an 
india-rubber bag containing air, 
while that at d is connected 
with an air-pump. 

My aim in this arrangement 
was to introduce at pleasure, into the portion of the tube 
between c and d, dry air, the common laboratory air, or air 




<1 






















132 


THE RELATION OF RADIANT HEAT 


artificially moistened. The point c, at which the air entered, 
was 18 inches from the source of heat S; the point d, at which 
the air was withdrawn, was 12 inches from the face of the pile. 
By adopting these dimensions, and thus isolating the central 
portion of the tube, one kind of air may with ease and cer¬ 
tainty be displaced by another without producing any agita¬ 
tion either at the source of heat on the one hand, or at the pile 
on the other. 

The tube A B being filled by the common air of the laboratory, 
and the needle of the galvanometer pointing steadily to zero, dry 
air was forced gently from the india-rubber bag through the 
cock c; the pump was gently worked at the same time, the dry 
air being thus gradually drawn towards d. On the entrance of 
the dry air, the needle commenced to move in a direction 
which showed that a greater quantity of heat was now passing 
through the tube than before. The dry air proved more 
transparent than the common air, and the final deflection 
thus obtained was 41 degrees. Here the needle stopped, and 
beyond this point it could not be moved by the further entrance 
of dry air. 

Shutting off the india-rubber bag and stopping the action of 
the pump, the apparatus was abandoned to itself; the needle 
returned with great slowness to zero, thus indicating a cor¬ 
respondingly slow diffusion of the aqueous moisture through 
the dry air within the tube. By working the pump the 
descent of the needle was hastened, and it finally came to rest 
at zero. 

Dry air was again admitted; the needle moved as before, and 
reached a final limit of 41 degrees; common air was again 
substituted, and the needle descended to zero. 

The tube being filled with the common air of the laboratory, 
which was not quite saturated, and the needle pointing to zero, 
air from the india-rubber bag was now forced through two 
U-tubes filled with fragments of glass wetted with distilled 
water. The common air was thus displaced by air more fully 
charged with vapour. The needle moved in a direction which 
indicated augmented absorption; the deflection obtained in this 
way was 15 degrees. 

These experiments have been repeated hundreds of times, and 


TO AQUEOUS VAPOUK. 


133 


on days widely distant from each other. I have also subjected, 
them to the criticism of various eminent men, and altered the 
conditions in accordance with their suggestions. The result 
has been invariable. The entrance of each kind of air is always 
accompanied by its characteristic action. The needle is under 
the most complete control, its motions are steady and uniform. 
In short, no experiments hitherto made with solids and liquids are 
more free from caprice , or more certain in their execution , than are 
the foregoing experiments with dry and humid air. 

The quantity of heat absorbed in the above experiments, 
expressed in hundredths of the total radiation, was found by 
screening off one of the sources of heat, and determining* the 
full deflection produced by the other and equal source of heat. 

By a careful calibration, repeatedly verified, this deflection was 
proved to correspond to 1,200 units of heat—the unit being, as 
before, the quantity of heat necessary to move the needle of the 
galvanometer from 0° to 1°. According to the same standard, 
a deflection of 41° corresponds to an absorption of 50 units. 
From these data we immediately calculate the number of rays 
per hundred absorbed by the aqueous vapour. 

1200 : 100 = 50 : 4-2. 

9 

An absorption of 4*2 per cent, was therefore effected by the 
atmospheric vapour which occupied the tube between the points 
c and d. Air perfectly saturated on the day in question gave an 
absorption of 5J per cent. 


§4. 


Radiation through Closed Tubes.—The Quantity of Heat absorbed 
proportional to the Quantity of Humid Air. 

These results were obtained in the month of September, and 
on the 27th of October I determined the absorption of aqueous 
vapour with the same tube when stopped with plates of rock- 
salt. Three successive experiments gave the deflections pro¬ 
duced by the aqueous vapour as 46*6°, 46*4°, 46*8°. Of this 
concurrent character are all the experiments on the aqueous 
vapour of the air. The absorption corresponding to the mean 




134 


THE RELATION OF RADIANT HEAT 


deflection here is 66. The total radiation through the exhausted 
tube was on this day 1085; hence we have 

1085': 100 = 66 : 6*1; 

that is to say, the absorption of the aqueous vapour of the air 
contained in a tube 4 feet long, was on this day 6 per cent, of 
the total radiation. 

The tube employed in these experiments was of brass, 
polished within; and it was suggested to me that the vapour 
of the moist air might have precipitated itself on the interior 
surface of the tube, thus diminishing its reflective power, 
and producing an effect apparently the same as absorption. 
In reply to this objection, I would remark that the air on 
many of the days on which the experiments were made was at 
least 25 per cent, under its point of saturation. It can hardly 
be supposed that air in this condition would deposit its vapour 
upon a polished metallic surface, against which, moreover, the 
rays from the source of heat were impinging. More than this, 
the absorption was exerted even when only a small fraction of 
an atmosphere was made use of, and found to be proportional 
to the quantity of atmospheric vapour present in the tube. The 
following table shows the absorptions of humid air at pressures 
varying from 5 to 30 inches :— 

Absorption per 100 


Pressures 
in inches 

/- 

Observed 


Calculated 

5 

16 


16 

10 

32 


32 

15 

49 


48 

20 

64 


64 

25 

82 


80 

30 

98 


96 


The third column here is calculated on the assumption that the 
absorption, within the limits of the experiment, is sensibly pro¬ 
portional to the quantity of vapour in the tube. The agreement 
with observation is almost perfect. It cannot be supposed that 
results so regular as these , agreeing so completely with those 
obtained with small quantities of other vapours , and even with 
small ’quantities of the permanent gases, can be due to the con¬ 
densation of vapour on the surface of the tube. When 5 inches 
were in the tube it had less than one-sixth of the quantity of 




TO AQUEOUS VAPOUR. 


135 


vapour necessary to saturate the space. Condensation under 
these circumstances is not to he assumed, and more especially 
a condensation which should produce such regular effects as 
those above recorded. 


§5. 

Radiation through the Open Air. 

The subject, however, is so important that I thought it worth 
while to make the following additional experiments:— 

C (fig. 12) is a cube of boiling water, intended for our source 


Fig. 12. 






of heat; Y is a hollow brass cylinder, 3*5 inches in diameter and 
7*5 inches in depth; P is the thermo-electric pile, and C the 
compensating cube; S is an adjusting screen, used to regulate 
the amount of heat falling on the posterior surface of the pile. 
The apparatus was entirely surrounded by boards, the space 
within being divided by tin screens into compartments which 
were loosely stuffed with paper or horsehair. The foimation of 
air-currents near the cube or the pile was thus prevented, and 
irregular motions of the external air were intercepted. A roof, 
moreover, was bent over the pile, and this vvas flanked by sheets 
of tin. The action here sought I knew must be small, and 
hence the necessity of excluding every disturbing influence. 
































136 


THE RELATION OF RADIANT HEAT 


The cylinder Y was first filled with fragments of qnartz moist¬ 
ened with distilled water. A rose burner r was placed at the 
bottom of the cylinder, and from it the tube t led to a bag con¬ 
taining air. The bag being subjected to gentle pressure, the 
air passed upwards amid the fragments of quartz, imbibing 
moisture from them, and finally discharged itself in the open 
space between the cube C and the pile. The needle moved and 
assumed a permanent deflection of 5 degrees, indicating that 
the opacity of the intervening space to the rays of heat was 
augmented by the discharge of the saturated air. 

The moist quartz fragments were now removed, and the vessel 
Y was filled with fragments of the chloride of calcium. The rose 
burner being, as before, connected with the india-rubber bag, 
air was gently forced up among the calcium fragments and 
discharged in front of the pile. The needle moved and assumed 
a permanent deflection of 10 degrees, indicating that the trans¬ 
parency of the space between the pile and source of heat was 
augmented by the presence of the dry air. By timing the 
discharges the swing of the needle could be augmented to 20 
degrees. Repetition showed no deviation from this result; the 
saturated air always augmented the opacity, and the dry air 
always augmented the transparency of the space between the 
source of heat and the pile. 



TO AQUEOUS VAPOUR. 


137 


§ 6 . 

# 

Application of Results to Meteorology—Tropical Rams — Cumuli — 
Condensation by Mountains—Temperatures at Great Eleva tions — 
Tliermometric Range in Australia , Tibet, and Sahara — Leslie’s 
Observations—Melloni on Serein. 

The power of aqueous vapour being thus established, meteoro¬ 
logists may, I think, apply the result without fear. That 10 
per cent, of the entire terrestrial radiation is absorbed by the 
aqueous vapour which exists within ten feet of the earth’s 
surface on a. day of average humidity, is a moderate estimate. 
In warm weather and air approaching to saturation, the absorp¬ 
tion would probably be considerably greater. This single fact 
at once suggests the importance of the established action as 
regards meteorology. I am persuaded that by means of it 
m any difficulties will be solved, and many familiar effects, which 
we pass over without sufficient scrutiny because they are 
familiar, will have a novel interest attached to them by their 
connexion with the action of aqueous vapour on radiant heat. 
While leaving these applications to be made in all their fulness 
by meteorologists, I would refer, by way of illustration, to one 
or two points on which the experiments seem to bear. 

And first it is to be remarked that the vapour which absorbs 
heat thus greedily radiates it very copiously. This fact must, 
I think, come powerfully into play in the tropical region of 
calms, where enormous quantities of vapour are raised by the 
sun, and discharged in deluges upon the earth. These have been 
assigned to the chilling consequent on the rarefaction of the 
ascending air. But if we consider the amount of heat liberated 
in the formation of those falling torrents, the chilling due to 
rarefaction will hardly account for the entire precipitation. The 
substance quits the earth as vapour, it returns to it as water; 
how has the latent heat of the vapour been disposed of? It has 
in great part, I think, been radiated into space. But the radia¬ 
tion which disposes of such enormous quantities of heat subse¬ 
quent to condensation, is competent, in some measure at least, 
to dispose of the heat possessed prior to condensation, and must 
therefore hasten the act of condensation itself. 

Aqueous vapour is a powerful radiant, but it is an equally 


138 


THE RELATION OF RADIANT HEAT 


powerful absorbent, and its absorbent power is a maximum when 
the body which radiates into it is vapour like itself. Hence, 
when the vapour first quits the equatorial ocean and ascends, 
it finds, for a time, above it a screen of its own substance, into 
which it pours its heat, and by which that heat is intercepted 
and in part returned. Condensation in the lower regions of 
the atmosphere is thereby prevented. But as the mass ascends 
it passes through successive vapour-strata, which Strachey has 
shown to diminish far more speedily in density than the asso¬ 
ciated strata of air, until finally our ascending body of vapour 
finds itself lifted above the screen which for a time protected 
it. It now radiates freely into space, and condensation is the 
necessary consequence. The heat liberated by condensation is, 
in its turn, spent in space, and the mass thus deprived of its 
potential energy returns to the earth as water. To what 
precise extent this power of aqueous vapour as a radiant comes 
into play as a promoter of condensation, I will not now inquire; 
but it must be influential in producing the torrents which are 
so characteristic of the tropics. 

The same remarks apply to the formation of cumuli in our 
own latitudes. They are the heads of columns of vapour 
which rise from the earth’s surface and are condensed to clohd 
at a certain elevation. Thus the visible cloud forms the capital 
of an invisible pillar of saturated air. Certainly the top of the 
column, piercing the sea of vapour which hugs the earth, and 
offering itself to space, must lose heat by the radiation from its 
vapour, and in this act alone we should have the necessity for 
condensation. The £ vapour plane 5 must also depend, to a 
greater or less extent, on the chilling effects of radiation. 

The action of mountains as condensers must, I think, be con¬ 
nected with these considerations. When a moist wind encounters 
a mountain-range it is tilted upwards, and condensation is no 
doubt to some extent due to the work performed by the expand¬ 
ing air; but the other cause cannot be neglected ; for the air 
not only performs work, but it is lifted to a region where its 
vapour can freely lose its heat by radiation into space. During 
the absence of wet winds the mountains themselves also lose 
their heat by radiation, and are thus prepared for actual surface 
condensation. We must indeed take into account the fact that 
this radiant quality of water is persistent throughout its three 


TO AQUEOUS VAPOUR. 


139 


states of exaggeration. As vapour it loses its heat and promotes 
condensation; as water it loses its heat and promotes congela¬ 
tion ; as snow it loses its heat and renders the surfaces on which 
it rests more powerful refrigerators than they would otherwise 
be. The formation of a cloud before the air which contains it 
touches a cold mountain, and indeed the formation of a cloud 
anywhere over a cold tract of land, where the cloud is caused by 
the cold of the tract, is due to the radiation from the aqueous 
vapour. The uniformly diffused fog which sometimes fills the 
atmosphere in still weather may be due to cold generated by 
uniform radiation throughout the mass, and not to the mixture 
of currents of different temperatures. The cloud by which the 
track of the Nile and Ganges (and sometimes the rivers of our 
own country) may be followed on a clear morning is, I believe, 
due to the chilling of the saturated air above the river by 
radiation from its vapour. 

Observation proves the radiation to augment as we ascend a 
mountain. Martins and Bravais, for example, found the lower¬ 
ing of a radiation-thermometer 5*7° C. at Chamouni; while 
on the Grand Plateau, under the same conditions, it was 13*4° 
C. The following remarkable passage from Hooker’s 4 Hima¬ 
layan Journals,’ 1st edit. vol. ii. p. 407, bears directly upon this 
point: 4 Prom a multitude of desultory observations I conclude 
that, at 7,400 feet, 125*7° or 67° above the temperature of the 
air, is the average maximum effect of the sun’s rays on a black- 

bulb thermometer.These results, though greatly above 

those obtained at Calcutta, are not much, if at all, above what 
may be observed on the plains of India [because of the dryness 
of the air.—J. T.]. The effect is much increased with the ele¬ 
vation. At 10,000 feet, in December, at 9 a.m. I saw the mer¬ 
cury mount to 132° [in the sun], with a difference [above the 
shaded air] of 94°, while the temperature of shaded snow hard 
by was 22°. At 13,100 feet, in January, at 9 a.m. it has stood 
at 98°, with a difference of 68*2°, and at 10 a.m. at 114°, with 
a difference of 81*4°, whilst the radiating thermometer outlie snow 
had fallen at sunrise to 0*7°.’ This enormous chilling is fully 
accounted for by the absence of aqueous vapour overhead. I 
never under any circumstances suffered so much from heat 
as in descending on a sunny day from the Corridor to the 
Grand Plateau of Mont Blanc. The air was perfectly stil], and 


140 


THE RELATION OF RADIANT HEAT 


the sun literally blazed against my friend Mr. Hirst and myself. 
We were hip deep in snow; still the heat was unendurable. Im¬ 
mersion in the shadow of the Home du Groute soon restoied oui 
powers, though the air of the shade was not sensibly colder than 
that through which the sunbeams passed. 

Without quitting Europe we find places where, even when the 
day temperature is high, the hour before sunrise is intensely 
cold. I have often experienced this even in Germany; and 
the Hungarian peasants, if exposed at night, take care, in hot 
weather, to prepare for the nocturnal chill. The range of tem¬ 
perature augments with the dryness, and an ‘ excessive climate 
is certainly in part caused by the absence of aqueous vapour. 

Regarding- Central Australia, Mr. Mitchell publishes ex- 
tremely valuable tables of observations, from which we learn 
that, when the days are at the same time calm and clear, the 
daily tliermometric range is exceedingly large. On the 2nd of 
March 1835 the temperature at noon was 68°, while that at 
sunrise next morning was 20°, showing a difference of 48°. The 
7th and 8th were also clear and calm; the difference between 
noon and sunrise on the former day was 38°, while on the latter 
it was 41°. Indeed between April and September a range of 
40° in clear weather was quite common—or more than double 
the amount observed in London at the corresponding season 
of the year. 

A freedom of escape similar to that from bodies at great ele¬ 
vations would occur at any other level, were the vapour removed 
from the air above it. Hence the withdrawal of the sun from 
any region over which the atmosphere is dry must be followed 
by quick refrigeration. This is simply an a priori conclusion 
from the facts established by experiment; but, I believe, all the 
experience of meteorology confirms it. The winters in Tibet are 
almost unendurable from this cause. The isothermals dip deeply 
from the north into Central Asia during the winter, the earth’s 
heat being wasted without impediment in space, and no sun 
existing sufficiently powerful to make good the loss. I believe 
the fact is well established that the desert of Sahara, which 
during the day is burning hot, is often extremely cold at night. 
This effect has been hitherto referred in a general way to the 
‘purity of the air: 5 but purity, as judged by the eye, is a very 
imperfect test of radiation, for the existence of large quantities 


TO AQUEOUS VAPOUR 


141 


of vapour is consistent with a transparent atmosphere. The 
purity really consists in the absence of aqueous vapour from 
those so-called rainless districts, which, when the sun is with¬ 
drawn, enables the hot surface of the earth to run speedily down 
to a freezing temperature. 

On the most serene days the atmosphere may be charged with 
vapour; in the Alps, for example, it often happens that skies 
of extraordinary clearness are the harbingers of rain. On such 
days, no matter how pure the air may seem to the eye, terrestrial 
radiation is arrested. And here we have the simple explanation 
of an interesting fact noticed by Sir John Leslie, which has 
remained without explanation up to the present time. This 
eminent experimenter devised a modification of his differential 
thermometer, which he called an JEtlirioscope. The instrument 
consisted of two bulbs united by a vertical tube, of a bore small 
enough to retain a little liquid index by its own adhesion. The 
lower bulb was protected by a metallic coating; the upper or 
sentient bulb was blackened, and was placed in a polished 
metal cup, which protected it completely from terrestrial 
radiation. 

6 This instrument,’ says its inventor, ‘ will at all times 
during the day and night indicate an impression of cold shot 

downwards from the higher regions.But the cause of 

its variations does not always appear so obvious. Under a. 
fine blue sky the 2Ethrioscope will sometimes indicate a cold of 
50 millesimal degrees ; yet on other days, when the air is equally 
bright , the effect is hardly 30°.’ It is, I think, certain that these 
anomalies were due to differences in the amount of aqueous 
vapour in the air, which escaped the sense of vision. Indeed, 
Leslie himself connects the effect with aqueous vapour by the 
following remark : c The pressure [? p>resence~\ of hygrometric 
moisture in the air probably affects the indications of the 
instrument.’ In fact, the absence or presence of vapour opened 
or closed an invisible door for radiation from the c sentient 
bulb ’ into space. 

The following observation in reference to radiation-experi¬ 
ments with Pouillet’s pyrlieliometer, now also receives its 
explanation. ‘ In making such experiments,’ says M. Schlag- 
intweit, c deviations in the transparency are often recognised 
which are totally inappreciable to the telescope or the naked 



142 


THE RELATION OF RADIANT HEAT 


eye, but which afterwards announce themselves in the presence 
of thin clouds/ &c. 

In his beautiful essay on dew, Wells gives the true explana¬ 
tion of the formation of ice in India, by ascribing the effect to 
radiation. I think, however, his theory needs supplementing. 
Given the same day-temperature in England as at Benares, 
could we, even in clear weather, obtain a sufficient fall of tem¬ 
perature to produce ice ? I think not. The interception of the 
calorific rays by our humid air would too much retard the chill. 
It is apparent, from the descriptions of the process, that a dry 
still air is the most favourable for the formation of the ice. 
The nights when it is formed in greatest abundance are those 
during which the dew is not copious. The flat pans used in 
the process are placed on dry straw, and if the straw become 
wet it must be removed. Wells accounts for this by saying 
that the wet straw is more dense than the dry, and hence 
more competent to transfer heat from the earth to the basins. 
This is hardly a satisfactory explanation; a better one seems to 
be that the evaporation from the moist straw, by throwing’ over 
the pans an atmosphere of aqueous vapour, checks the radiation 
and thus diminishes the cold. 

Melloni, in his excellent paper c On the Nocturnal Radiation 
of Bodies/ gives a theory of serein , an excessively fine rain 
hich sometimes falls in a clear sky a few moments after sunset. 
Several authors, he says, attribute this effect to the cold resulting 
from radiation of the air, during the fine season, immediately on 
the departure of the sun. ‘ But/ writes Melloni, ‘ as no fact 
is ^ et know n w hich distinctly proves the emissive power of pure 
transparent elastic fluids, it appears to me more conformable to 
the principles of natural philosophy to attribute this species of 
rain to the radiation and subsequent condensation of a thin veil 
of vesicular vapour distributed through the higher strata of the 
atmosphere. 5 Now, however, that the power of aqueous vapour 
as a radiant is known, the difficulty experienced by Melloni dis¬ 
appears. The former hypothesis, however, though probably 
correct in ascribing the effect to radiation, was incorrect in 
ascribing it to the radiation of ‘ the air .’ 

Dr. Hooker encourages me to hope that this newly discovered 
action may throw some light on the formation of hail. The 

* Taylor’s Scientific Memoirs, vol. v. p. 551. 


TO AQUEOUS VAPOUR. 


143 

wildest and vaguest theories are afloat upon this subject. But 
the same action which produces serein must, if augmented, freeze 
the minute rain, and the aggregation of the small particles thus 
frozen would form liail. Many kinds of hail that I have had an 
opportunity of examining could not be due to the freezing of 
d ro P g °f water, each hailstone being merely the ice of the drop. 
The ‘ stones ’ are granular aggregates, the components of which 
may be produced by the chill of radiation. I will not, how¬ 
ever, dwell further on this subject, but will now commit the 

entire question to those who are more specially qualified for its 
pursuit. 






























































IV. 


OX THE PASSAGE OF RADIANT HEAT THROUGH 

DRY AND HUMID AIR. 


ANALYSIS OF MEMOIR IY. 


VTith Professor Maim ns's visit to London. I thought the points of difference 
between us settled. The experiment with the open tube had evidently irn- 
pressc-d him. and, with a view to repeating it, on his return to Berlin, he 
mounted an apparatus similar to mine. * The result of this experiment, he says, 

* was so surprisinz. and so little in accordance with what I had found by other 
processes, that when I reached home I determined to repeat it.’ He describes 
his apparatus and mode of usir.ir it. and thus states his results:—* Y ith this 
arrangement I got, on allowing dry or moist air to flow through the tube, 
defleeti ns of the mdvanometer which corresponded to those described by Pro¬ 
fessor TyndalL But I did not always get them: and what particularly surprised 
me was that the deflection of the needle did not correspond to an absorption of 
heat bv its passage through moist air. but that, on the contrary, when the 
moi?t air was passed through the tube, the face of the pile turned towards the 
tube was found to be most heated. In order to clear up the already mentioned 
uneertaintv of the experiment. I have repeated the blowing in of dry and moist 
air manv hundred rimes: but in no single case was the deflection such as to 
indicate a greater absorption by moist air.’ 

‘It would be out of place,’ he continues. ‘ to relate the numerous experiments 
which were undertaken, portly in order to mike myself master of the phenomena, 
and partly in order to explain the surprising contradiction between my results 
and the conclusions which Professor Tyndall has drawn from his experiments. 
I found, in the hrst place, that the deflection took place only when the air was 
driven in with a certain amount of force. It was found, further, that when the 
air was pressed in continuously, the deflection of the galvanometer was not 
maintained constant, but that the instrument gradually returned to its position 
of equilibrium. Hence it resulted that the air did not cause the deflections by 
absorption. I suspected that possibly moisture might be condensed on the in¬ 
ternal surface of the tube, and that the heating effect might be thus produced ; 
but this supposition was likewise found to be erroneous. It appears, on the 
other hand, that the phenomenon is caused bv the absorption which takes place 
at the surface of the pile itself.’ 

Professor Magnus then proves in the most satisfactory manner that the 
deflections observed in his experiments were whollv due to the condensation of 
the aqueous vapour of tne moist air on the surface of his pile, and its subse¬ 
quent evaporation by dry air. ‘ Y e see,’ he continues, ‘ by these results, how 
little fitted air is. while in motion, for experiments as to its power of absorption.’ 

Had Tr trS.or Magnus allowed himself the time requisite for testing it, he 
woul d not. I am persuaded, have offered this solution of the differences existing 
between us. ‘Air in motion,’ employed as long experience has taught me to 
employ it. yielded at the time here referred to perfectly certain and concurrent 


ANALYSIS OF MEMOIR IV. 147 


heat or with 
present case 
Memoir III. 


results. Is or were these results, as supposed by Professor Magnus, dependent 
on any accident as to the dimensions of the apparatus. 

Numerous passages in these memoirs show how fatal# to accuracy I regarded 
the bringing of air or any other gas into contact either with the source of 
the surface of the pile ; it was not therefore likely that in the 
I should shut my eyes to so obvious a source of error. In 
(page 113 ) I actually referred to it. Special experiments, more- 
o\er, had assured me that no air from the experimental tube came near the 
pile; and in a thousand trials I never had the slightest difficulty in obtaining 
the results contro\ erted by the foregoing experiments. They were perfectly 
piecise, closely concurrent, and always attainable. Nevertheless, on reading 

I lofessoi Magnus s paper I went once more over the ground, and verified all 
my former statements. 

As, theiefore, I could not add to my own certainty, and as the evidence of an 
independent observer seemed desirable, I handed over the whole apparatus to 
Dr. Frankland, who minutely tested every point involved in, or arising out of, the 
objection of Professor Magnus. He showed that the source of error signalised 
above had nothing to do with my experiments, making it evident that its in¬ 
troduction could only arise from instrumental defects. Professor Frankland 
expresses his conclusions in these words :— c After a careful scrutiny, I have 
been unable to detect any source of fallacy in these experiments; they there¬ 
fore appear to me to prove conclusively that obscure radiant heat passes much 
more readily through dry than through moist air. In conclusion, I cannot but 


express my surprise and admiration at the precision and sharpness of the indi¬ 
cations of your apparatus. Without having actually worked with it I should 
not have thought it possible to obtain these qualities in so high a degree in 
determinations of such extreme delicacy, and which are so well known to be 
exposed to numerous sources of derangeme.nt,’ * 

In the concluding portion of this memoir the effect of bringing the gases 
iuto contact with the source of heat is briefly considered, and the effect of 
contact with the pile is more fully developed. It is shown that deflections the 
same in kind as those which Professor Magnus ascribes to absorption, must 
be produced by the contact of the air whether its temperature be higher or lower 
than that of the pile. 

The disturbance arising from the dynamic heating of the air, and of all sur¬ 
faces against which it impinges, is also dwelt upon and illustrated. 


In the communication of Professor Magnus above cited, a remark occurs which 
merits a word of explanation. 1 Besides the defect,’ he writes, ‘ arising from the 
hygroscopic character of the rock-salt plates, Professor Tyndall's method labours 


* In a brief communication inserted in PoggendorfF’s Annnlen , and translated in the 
Philosophical Magazine , vol. xxvii. p. 249, Professor Magnus thus speaks of Frankland’s 
confirmation :—‘ Dr. Tyndall has had his experiments repeated by Dr. Frankland, in order, 
as he says, to prove that he had not mistaken cold for hot and hot for cold. Such a con¬ 
firmation was, in my opinion, unnecessary. I have not implied an error of that kind, but 
have only said that, in repeating Dr. Tyndall’s experiments, it has not even once happened 
to me to obtain the same result as he did.’ Surely that an investigator of such acknow¬ 
ledged skill and caution was not only unable to obtain my results, but, in ‘many hundred ’ 
experiments, had obtained others diametrically opposed to mine, was a sufficient justificar- 
tion of the desire to see my experiments verified. 


148 


ANALYSIS OF MEMOIR IV. 


under another difficulty. . . . The value obtained for dry air formed the unit 
for the determination of the other gases, all of which were compared jn the 
same way with the vacuum. Therefore the smaller the difference between the 
dry atmospheric air and the vacuum, the greater the apparent absorptive power 
of the other gases. Hence, if this difference were to be equal to nothing, the 
absorption of the other gases would come out infinitely great.’ 

I would ask it to be remembered that my object in these inquiries was not 
to follow the track of my eminent predecessors, who made radiant heat the 
primary object of their thoughts, but rather to employ radiant heat as an explorer 
of molecular condition. What I aimed at specially in these first memoirs was to 
bring clearly into view the astonishing change in the relations of the luminife¬ 
rous aether accompanying the act of chemical combination. I wanted to show 
the physical significance of an atomic theory which had been founded on purely 
chemical considerations. By making the substance of feeblest action my 
standard, and referring all the others to it, this purpose was carried out in the 
most direct and simple way. My tables, in fact, resemble those of the atomic 
weights, in which hydrogen, the lightest atom, is employed as a standard. 


IV. 


ON THE PASSAGE OF RADIANT HEAT THROUGH 

DRY AND HUMID AIR.* 

It is known to tlie reader that Professor Magnus and 
myself have arrived at different conclusions regarding the 
action of dry air, and of the aqueous vapour diffused through¬ 
out our atmosphere, on radiant heat. Last autumn I had 
the pleasure of meeting my eminent friend in London; and 
soon after his arrival it was agreed upon between us to 
subject the points on which we differed to a searching ex¬ 
amination. We accordingly met on several occasions in the 
laboratory of the Royal Institution, where every result that 
I had previously announced was reproduced in the presence 
of Professor Magnus. Facts were placed before him which he 
professed his inability to explain ; but, like a cautious philo¬ 
sopher, he reserved his opinion. It was, however, proved that 
the results observed by us in common could not be ascribed 
to any defect of method or error of observation which it was 
then possible to point out. I wished very much to subject the 
most recent experiments of Professor Magnus to a similar 
examination, and he evinced an equal desire to show them to 
me. He began his arrangements, but it was not my good 
fortune to see them accomplished. In fact, coming to London 
as a visitor to the International Exhibition, the numerous other 
claims upon his time and attention were amply sufficient to 
prevent him from carrying out his own wishes and gratifying 
mine. 

In the latest number of Poggendorff’s 4 Annalen,’ Professor 
Magnus has published a paper 4 On the Diathermancy of Dry 
and Moist Air,’ a translation of which is printed in the 


* Philosophical Magazine, July 1863. 


150 


THE PASSAGE OF RADIANT HEAT 


6 Philosophical Magazine ’ for July I 860 . From it I learn that 
the experiments on atmospheric vapour which struck him 
most in London were those performed with a tube open at 
both ends. The results thus obtained were so opposed to 
those obtained in another way by himself, that he returned 
to Berlin resolved to repeat my experiments. The paper just 
referred to contains an account of his researches, and a pro¬ 
posed explanation of my results. 

Operating with an open tube, he displaced with a pair 
of bellows dry air by moist and moist air by dry, and ob¬ 
tained, though not always, deflections corresponding to mine. 
But he w r as particularly surprised to find that the direction in 
which the needle moved when moist air was blown into the 
tube, indicated, not a withdrawal of heat from the thermo¬ 
electric pile, but an augmentation of heat. When dry air was 
forced into the tube, the deflection observed, instead of indi¬ 
cating that a greater amount of heat fell upon the pile, showed, 
on the contrary, that the pile was chilled. These effects, he 
urges, are due to the absorption of aqueous vapour by the 
lampblack coating of the thermo-pile. This absorption, when 
moist air was blown against the instrument, rendered heat free; 
when dry air, on the other hand, was forced against it, the 
evaporation of the condensed vapour chilled the pile, and the 
deflection due to cold was observed. He wishes it, in short, 
to be inferred that in my experiments cold has been mis¬ 
taken for heat, and heat for cold, and that effects have been 
ascribed to absorption which are really due to the condensation 
and evaporation of aqueous moisture at the surface of my 
thermo-electric pile. 

To commit such an error, and to persist in it so long, would, 
1 fear, leave me lilfcle claim to confidence as an experimenter. 
But the truth is that some years have ^elapsed since I became 
acquainted with the facts now urged against me by Professor 
Magnus. Experimenting years ago on dry and moist air with 
tubes which had been coated inside by lampblack or lined with 
blackened paper, I found, when moist air was introduced, the 
radiation from the interior surface so energetic as to compel me 
tp abandon the coating. The promptness and energy with which 
these effects of condensation and evaporation are produced are 
remarkable. Dry air urged against the face of my pile on a 


THROUGH DRY AND HUMID AIR. 


151 


day of average humidity drives the needle of my galvanometer 
through an arc of 196 degrees, and keeps it for a time pointing 
to nearly 90°, from which, while the air-current continues, it 
gradually sinks to zero. On simply stopping the current of dry 
air, the needle swings quickly to the other side of zero, passing 
through an arc of 120 degrees, this large deflection being pro¬ 
duced by the sudden re-absorption of the atmospheric vapour 
when the dry air is intercepted. Air artificially moistened pro¬ 
duces still larger deflections. 

Such effects were well known and duly guarded against. 
Indeed, it is to me interesting to notice my own experience in 
this inquiry reproduced years subsequently in the experience of 
Professor Magnus. I never had the least doubt of the correct¬ 
ness of his results; but, for the most part, they have absolutely 
nothing to do with mine. We are equally successful in our 
efforts. His object, for example, is to bring the hygroscopic 
character of rock-salt into strong relief,, and he succeeds in 
wetting the plates; my object is to avoid this source of dis¬ 
turbance, and I am equally successful. He, by blowing vigorously 
into his tube, urges the air against the face of his pile, and 
obtains the effects due to condensation and evaporation; I, by 
operating cautiously and permitting the air to enter the tube so 
slowly and at such distances from the source and from the pile 
that neither of them is affected by it, obtain the effects due to 
absorption. One great feature of the case, however, is, that 
while the results of Professor Magnus have been known to me 
for years, and while I can produce them on a large scale at 
any moment, he has not yet succeeded in reproducing mine. 

4 Never/ he writes, 4 in a single instance has the deflection indi¬ 
cated a greater absorption by the humid air.’ 

After reading the last paper of Professor Magnus, I felt that 
it would be useless on my part to reiterate what had been 
already so often affirmed, and I therefore wished to subject my 
experiments to the scrutiny of an independent observer. Mr. 
Faraday had already seen those experiments, and it was purely 
my reluctance to give him trouble that prevented me from 
asking him to witness them again. Next to him I could hardly 
find a man whose testimony on such a subject will have greater 
weight than that of my colleague, Dr. Frankland; and he, at my 
request, kindly undertook to satisfy himself upon the points at 


152 


THE PASSAGE OF RADIANT HEAT 


issue. I mounted the apparatus, and left it entirely in liis 
hands; and he has favoured me with the following account of 
his observations:— 

‘ My dear Tyndall, —At your request I have made a 
number of experiments on the comparative transcalency of 
common air, and of air deprived of its moisture by contact with 
monohydrated sulphuric acid. The apparatus which I used was 
that described by you in the ‘ Philosophical Transactions 9 for 
December 1862. It was exclusively under my own control; 
and I arranged the details of manipulation in such a manner as 
appeared to me best calculated to eliminate all sources of error. 
My mode of operating was as follows:—The brass tube open 
at both ends, formed the conduit for a portion of the thermal 
radiation from the source of heat. These heat-rays, after 
passing through the tube, traversed several inches of interven¬ 
ing air-space before entering the cone of the thermo-electric 
pile, where they produced their effect, in opposition to that 
arising from another constant source of heat affecting the 
opposite face of the pile (the compensating cube). The dif¬ 
ferential action was indicated as usual by a delicate galvano¬ 
meter. These arrangements being once for all made, I was 
able by means of an air-pump to introduce at pleasure into the 
tube either the ordinary air of the laboratory, dry air, or air 
rendered moist by passage over extensive surfaces of wet glass. 
At the commencement of the experiments the tube was of course 
full of the' common air of the laboratory; the needle of the gal¬ 
vanometer marked 42°, and remained steady for a quarter of 
an hour within a degree of that point. I now interposed in the 
path of the rays entering the brass tube a sheet of tin-plate ; the 
needle at once bounded from 42° up to 90°. It was thus evident 
that any obstruction to the passage of the rays of heat through 
the tube, or, in other words, any cooling of that face of the pile 
which was turned towards the tube, would be indicated by an 
increased deflection of the needle on the same side of zero, which 
I will call the — side, whilst a heating of the same face of the 
pile would be attended by a diminished deflection, or even by 
a passage of the needle to the opposite or + side of zero. The 
following are the results which I observed:— 


THROUGH DRY AND HUMID AIR. 


153 


Permanent 

Tube filled with deflection of need’* . 

* O 

Common air . . . . . . . . . . . —42 


Air dried by contact with monohydrated sulphuric acid and intro¬ 
duced gently into the open tube. 

Common air 
dry air 


Common air gently drawn in by air-pump ..... — 43'3 

Common air gently blown in from caoutchouc bag . . . . — 45 

Same air gently blown in from caoutchouc bag, but dried by passage I + ^ 
over sulphuric acid ........ j 

Air from same bag, but not dried ....... — 46‘5 

Air of laboratory . . . . . . . . . . — 42 

Air dried and introduced as before . . . . . . +14 

Air dried as before with sulphuric acid, but afterwards passed I 
over fragments of glass moistened with water . . . ) 

Common air gently drawn in by air-pump . . . . . — 42'5 


c At tlie conclusion of tlie experiments I found that the 
deflection due to the total radiation was 80°*2. 

6 1 also saw the following experiments made by yourself when 
the ends of the brass tube were closed by plates of rock-salt:— 

Permanent deflection 
of needle 

O 

Tube filled with dry air . . .+ 7 

After exhaustion of tube . . — 3 

After admission of laboratory air . — 42 


which had spontaneotisly displaced the foregoing ^ 

» . . . . . . . . . s 


43 


c Rain was falling at the time these last determinations were 
made, and the air was very moist. On removing the plates of 
rock-salt from the tube they appeared to be quite dry; and 
after being breathed upon, the film of moisture soon disappeared 
and they recovered their previous lustre. I ought perhaps to 
mention that these experiments are not selected, they are the 
only ones I have made upon the subject, and they were per¬ 
formed in the sequence given above: after a very careful scrutiny 
I have been unable to detect any source of fallacy in them, and 
they therefore appear to me to prove conclusively that obscure 
radiant heat passes much more readily through dry than through 
moist air. 

c In conclusion, I cannot but express my surprise and admi¬ 
ration at the precision and sharpness of the indications of your 
apparatus. Without having actually worked with it I should 
not have thought it possible to obtain these qualities in so high 
a degree in determinations of such extreme delicacy, and which 





154 


THE PASSAGE OF RAD I AST HEAT 


are so well known to be exposed to numerous sources of de¬ 
rangement. 

4 Believe me, yours very truly, 

4 E. Frankland. 

* Royal Institution, 

June 19, 1863.’ 

4 P.S.—Since writing the foregoing letter, I have repeated the 
experiments there recorded without any source of heat at either 
end of the pile, in order to ascertain whether the introduction 
and withdrawal of dry air at all affected the galvanometer. 
The tube was first full of the common air of the laboratory, and 
the needle remained steadily at -f 12*5 for a quarter of an hour. 
A current of moistened air was now drawn through the tube 
for ten minutes in precisely the same manner as when the two 
sources of heat were employed, the needle being closely watched 
during the whole time. It oscillated between +12 and +13, 
but never passed these limits on either side. The current was 
now interrupted and the needle closely watched for five minutes : 
it remained perfectly steady at 12*5. A current of dried air 
was now conveyed through the tube for ten minutes ; the needle 
oscillated as before between 12° and 13°. 

4 Thus far I operated on the air exactly as in the experiments 
recorded in the foregoing letter. I then quadrupled the velo¬ 
city of the current through the tube, introducing in the first 
place dry air: the needle in a first experiment moved 6 degrees 
in the direction of cold; but on repeating the experiment with 
both dry and moist air no effect whatever was produced. I 
now removed the tube and delivered a gentle current of dry 
air into the cone of the pile ; immediately the needle moved 90° 
in the direction of cold. The current was continued uninter¬ 
ruptedly for ten minutes,' during which time the needle gradually 
returned to nearly its original position. The current of dry air 
being now stopped, the needle moved 40° in the direction of 
heat , returning again gradually and slowly to its normal 
position. The same temporary deflection for heat was also 
produced in an exalted degree when the dry current was 
immediately succeeded by a moist one. 

4 These supplementary experiments lead me to the following 
conclusions:— 

4 1st. The gentle currents of air which were caused to flow 
through the tube in the experiments detailed in my letter did 


THROUGH DRY AND HUMID AIR. 


L k * 

DO 


not in any way disturb tbe results of those experiments, neither 
would they have done so in any material degree even had their 
velocity been quadrupled. 

2ndly. The impact of air drier than that previously in contact 
with the pile cools that face of the instrument with which it 
comes in contact, whilst the like impact of moister air produces 
the opposite effect. 

‘ 3rdly. It is, however, impossible to confound the effects 
obtained in the above experiments on transcalency with those 
produced by the impact of dry and moist air upon the face of 
the pile, because in the first place the former are permanent, 
whilst the latter are essentially transitory ; and in the second 
place the deflections due to the impact of dry or moist air 
against the face of the pile are always in the opposite direction 
to those obtained by the interposition of the same kind of air 
in the path of radiant heat. Thus, if the heat-rays falling upon 
one face of the pile be made to traverse dry air, the needle will 
move in the direction of heat, but if the apparatus be so 
arranged as to cause the dry air to impinge upon the face of 
the pile, the effect due to the greater transcalency of the dry 
air would be at first more or less neutralized, or even altogether 
overborne, by the cooling influence due to evaporation at the 
surface of the pile so brought into contact with dry air.—E. E. 

‘ Royal Institution, 

June 20, 1863.’ 

In my remarks on the experiments of Professor Magnus, 
I had pointed out two sources of error in the method 
which he employed. One of these was the bringing of the 
cold gas to be examined into direct contact with his source of 
heat: and the other was the bringing of the same gas into 
direct contact with the face of his thermo-electric pile. In his 
last paper he urges, in reference to the first point, that my 
objections do not apply to his apparatus, because in it the 
column of air is heated at the top. This argument would be 
strictly valid if the heat could be applied with perfect uniformity 
to a perfectly horizontal plane, but in practice such perfection 
is not attainable. The top of Professor Magnus’s recipient is 
dome-shaped, even where it is in perfect contact with the source 
of heat, while beyond the limits of this contact—that is to say, 


156 


THE PASSAGE OF RADIANT HEAT 


down the sides of the recipient—it is propagated more or less 
by conduction. Indeed Professor Magnus himself states that a 
portion of the heat effective in his experiments is derived from 
the glass thus warmed. 4 The heating of the thermometer,’ he 
writes. 4 although due only to conduction and radiation, involves 
a very complicated process. Besides the direct heating through 
conduction and radiation, reflexion also tabes place at the inner 
surface of the vessel. Further, tlie portions of the surface adja¬ 
cent to the vessel of boiling water are heated by conduction , and 
also radiate heat against the thermometer,’ * I have italicised 
the most important part of the passage. Now air in contact 
with such a surface is substantially in the same condition as in 
my front chamber, and such air, as I have shown, must more or 
less diminish the temperature of the surface exposed to it. If 
Professor Magnus fails to detect this with his new apparatus, it 
can, I think, only be due to its lack of the requisite delicacy. 
Without the actual numbers no safe opinion can be formed upon 
this point; the probability is that his total heat is so small that 
the lowering of the temperature of his source of heat by the 
admission of air into contact with it becomes infinitesimal. 

An important difference between Professor Magnus and 
me consists in the high absorptive power which he ascribes to 
dry air. His absorption, in fact, is more than 140 times mine. 
I would here bespeak the reader's attention to an examination 
of the conditions in which Professor Magnus places his instru¬ 
ments. From his last figure, and also from a passage of his 
paper, it is to be inferred that in his recent experiments the air has 
free access to the two faces of fiiis pile, the axis of which is 
vertical. The upper face is furnished with a conical reflector, 
while the lower one is provided with one of the cylindrical tubes 
which usually accompany the instrument. It will repay us to 
reflect for a moment on the processes involved in this arrange¬ 
ment. Professor Magnus keeps the space which contains his pile 
at a constant temperature of 15° C. Let us suppose the two faces 
of his pile to be at the same temperature, the radiation from the 
source being suspended, and the space around the pile a vacuum. 
Suppose, in the first instance, the temperature of the air outside 
to be lower than that of the pile, that the pile, in other words, is 
a warm body in comparison with the air ; what will be the effect 

* PoggendorfFs Annaten, vol. cxii. p. 544. 


THROUGH DRY AXD HUMID AIR. 


157 


of admitting the air into tlie vessel ? * Manifestly on the upper 
face of the pile will rest a column of air, heated at its bottom 
by the suiface on which it rests ; convection wil] immediately set 
in, and heat will be continually abstracted from the face of the 
pile. At the lower face, on the contrary, an equal abstraction 
does not take place; for the air once warmed remains in con¬ 
tact with the face of the pile, convection here being almost nil. 
Thus a less amount of heat is abstracted from the lower than 
from the upper face of the pile, and hence the instrument, 
which before the entrance of the cool air produced no current, 
must, in virtue of the different action of this air on its 
two opposite faces, generate a current similar to what would be 
produced by the direct heating of the lower face of the pile. 

A moment’s reflexion suffices to prove that precisely the 
same deflection is obtained when the external air is hotter than 
the pile. Supposing, as before, the temperature of both faces 
to be the same at the commencement, the needle of the asso¬ 
ciated galvanometer being at zero. When the warm air enters 
it is chilled by the uj>per face of the pile, contracts, and remains 
in contact with that face, forming in fact a 'pool of heavy air at 
the bottom of the reflector. The air chilled by the opposite 
face of the pile falls by its weight; its place being supplied by 
fresh warm air. It is therefore evident that the lower face of the 
pile will in this case be more heated by the air than the upper 
one; and hence we infer that whether the external air be colder 
than the pile, or hotter than the pile, the same galvanometric 
effect follows its introduction into the vessel. 

Instead of supposing the pile to be in the first place of uniform 
temperature, let us imagine it exposed to the radiation from the 
source of heat. This makes the upper face warmer than the 
under one, and produces a deflection commensurate with the 
difference of temperature of the two faces. Let air now enter: 
it is manifest from the- foregoing analysis that, whether 
this air is colder than the pile or hotter, its effect will be to 
render the lower face relatively warmer, and thus to diminish 
the deflection. If, moreover, the air be of the exact tempera¬ 
ture of the upper face, it will warm the under one;* if of the 


* The reader, if he wish, may see a drawing of Professor Magnus’s'apparatrs in 
the Philosophical Magazine for 1863, vol. xxvi. 


158 


THE PASSAGE OF RADIANT nEAT 


exact temperature of the under face it will chill the upper one. 
If its temperature be the mean temperature of the mass of the 
pile, it will chill the upper face and warm the lower one at the 
same time. No matter, then, wliat the temperature of the air 
may be when it enters the vessel, the effect of its contact with the 
pile is to diminish the defection due to the radiation from the source 
of heat, and thus produce the same galvanometric effect as a true 
absorption. How Professor Magnus releases his pile from this 
apparently inevitable action he does not inform us ; and how 
he can distinguish between this effect, in which absorption has 
absolutely nothing to do, and one of real absorption, I am at a 
loss to imagine. 

His apparatus will enable him to make this exj)eriment in a 
far more unexceptionable manner. Let him place a second plate 
of salt across his tube immediately above his pile, and thus 
isolate it from the air which he intends to examine. He will then 
obtain the almost pure effect of radiation. Professor Magnus has 
actually made this experiment, and the result, expressed in his 
own words, is 6 a hardly perceptible difference between dry air 
and a vacuum.’ 

It is scarcely necessary to repeat what I have already stated 
regarding the dynamic heating of the pile when the air enters. 
Professor Magnus never once refers to this effect, though he 
does refer, for the first time, in his last paper to the chilling 
consequent on pumping out. Had his apparatus been suffi¬ 
ciently delicate, the effect to which I refer must have long 
ago attracted his attention. Some conception of its mag¬ 
nitude may be formed from the following quotation from a 
paper laid before the Royal Society on the 18th of this month:— 

‘A brass tube 3 feet long and very slightly tarnished within 
was used for dynamic radiation. Dry air on entering the tube 
produced a deflection of 12 degrees. The tube was then polished 
within and the experiment repeated : the deflection by dry air 
was instantly reduced to 7-5 degrees. 

‘ The rock-salt plate at the end of this tube was removed, and 
a lining of black paper 2 feet long was introduced within it. 
The tube was again closed, and the experiment of allowing dry 
air to enter it was repeated. The deflections in three successive 
experiments rose from 7*5° to 

80 °, 81 °, 80 °, 


THROUGH DRY AND IIUMID AIR. 


159 


and this result might be obtained as long as the lining was per¬ 
mitted to remain within the tube. 

‘ The plate of rock-salt was again removed, and the length of 
the lining was reduced to a foot; the dynamic radiation on the 
entrance of dry air produced in three successive experiments 
the deflections 

76°, 74°, 75°. 

c The plate was again removed and the lining reduced to three 
inches in length; the deflections obtained in two successive ex¬ 
periments were 

66°, 65°. 

‘Finally, the lining was reduced to a ring only 1^ inch in 
width; the dynamic radiation from this small surface gave, in 
three successive trials, the deflections 

60°, 56°, 56-0°. 

c The lining was then entirely removed; and the deflection 
instantly fell to 

7*5°. 

‘ In the foregoing experiments the lining was first heated by 
the collision of the air, and it then radiated its heat through a 
thick plate of rock-salt against the pile. The effect of the heat 
was enfeebled by distance, by reflexion from the surface of the 
salt, and by partial absorption. Still we see the radiation thus 
weakened competent to drive the needle almost through the 
quadrant of a circle. Suppose, instead of being thus separated 
from the lining, the face of the pile itself [as in the experiments 
of Professor Magnus] to form part of the inner surface of the 
tube, receiving there the direct impact of the particles of air; 
of course the deflections then obtained would be far greater 
than the highest of those above recorded. I do not doubt the 
possibility of causing the needle of my galvanometer, subjected 
to such an action, to swing through an arc of 1,000 degrees; 
and it is my reluctance to derange the magnetism of my needle 
that prevents me from making the experiment.’ * 

Professor Magnus refers to the agreement which subsists 

* When the pile was placed entirely within the tube (as Professor Magnus places 
it), a single stroke of the pump in exhausting drove the needle through an arc of 


160 


THE PASSAGE OP RADIANT HEAT 


> k 

between bis results and mine in the case of tbe more power¬ 
fully acting gases, in proof of the correctness of bis mode of 
experiment. Tbe agreement, however, is not sucb as to warrant 
tbe conclusion drawn from it. The case may be illustrated by 
reference to a delicate chemists’ balance as compared with one 
of those used in common life. Weighing pounds , both balances 
would roughly agree, but in weighing milligrammes the coarser 
balance would infallibly fail. I think it vain to expect a correct 
determination in any case requiring great delicacy with the 
apparatus which Professor Magnus employs.* 

He again refers to the hygroscopic character of rock-salt. 
This is admitted. His experiments on this substance are quite 
correct; but they have no bearing upon mine. During our joint 
experiments, and while the humid air, whose absorption pro¬ 
duced a deflection of 43 degrees of my galvanometer, was still 
in the experimental tube, the rock-salt, plates were detached 
and placed in his hands. He saw no moisture, and he expressed 
himself satisfied that there was none. Professor Magnus finds 
another difficulty in the fact that I make air my unit, and 
refer the action of all other gases to this unit. There is, I 
submit, no more e difficulty ’ here than in the tables of atomic 
weights, where hydrogen is taken as the unit. My object was, 
and is, to make radiant heat an explorer of molecular condition ; 
and my results seem to me more instructive and emphatic as 
now presented than if I had followed the common method 
pursued by Professor Magnus. The difficulty referred to does 
not touch the method of experiment at all, but merely my 
way of presenting the results. 

I may add that, in a paper recently presented to the Poyal 
Society, the action of all the vapours which I have examined 
is compared with that of the liquids from which these vapours 
are derived. The order of absorption of vapours and liquids is 
precisely the same.f At the bottom of the list stands water, as 
the most opaque liquid examined. It would form a most remark¬ 
able exception to what, so far as I can see, is a general law , if 
the vapour of this liquid proved so ineffectual as the experi¬ 
ments of Professor Magnus make it. One word with reference 

* This remark, I fear, -was displeasing to Professor Magnus. His retort was warm. 
See Philosophical Magazine , 1864, vol. xxvii. p. 250. 

f See Section 9 of Memoir V. and Section 5 of Memoir YI. of this collection. 


THROUGH DRY AND HUMID AIR. 


161 


to the importance of this subject. In a certain sense Professor 
Magnus is quite right in rating it low. It derives its im¬ 
portance from the fact that aqueous vapour is everywhere 
present in our atmosphere, and that, for the future, the proved 
action of this vapour must form one of the chief foundation- 
stones of the science of meteorology. 

Royal Institution, 

June 19, 1863. 


11 


to 









Y. 

ON THE ABSORPTION AND RADIATION OF nEAT 
BY GASEOUS AND LIQUID MATTER. 


ANALYSIS OF MEMOIR Y. 


Melloni determined the transmission of radiant heat through liquids and 
solids of different thicknesses : in the following investigation the same is ellected 
for gases and vapours. 

A new experimental arrangement is described, which permits of varying the 
thickness of the gaseous layers between the limits of one-hundredth of an inch 
and forty-nine inches; or in the ratio of 1 : 4900. With the stronger gases 
even the thinnest of these layers is shown to yield a measurable action. 

The influence of a diathermanous envelope upon the temperature of a planet 
has been more than once adverted to in these memoirs. It is here proved 
experimentally that a layer of olefiant gas only two inches in thickness 
wrapping the earth, and allowing comparatively free passage to the solar rays, 
would intercept at least 33 per cent, of the terrestrial radiation. 

It is also shown that an envelope of sulphuric-ether vapour two inches thick 
would stop at least 35 per cent, of the terrestrial radiation. In connexion with 
these results, the possible action of aqueous vapour is adverted to. 

Experiments are then described where the experimental tube is divided into 
two chambers by a rock-salt partition ; the radiation from both chambers, taken 
singly and together, being determined for both gases and vapours. The dividing 
rock-salt is made to occupy different positions within the tube, so as to render 
the two chambers sometimes of equal, and sometimes of unequal length. 

The sifting of the heat by one chamber, and its influence on the transmission 
of the heat through the other, are pointed out. 

New experiments on Dynamic Radiation are described, the arrangement 
permitting the dynamically heated gas or vapour to radiate through a vacuum, 
through a column of its own substance, or through a column of any other gas 
or vapour. The influence of coincidence in vibrating period is further illus¬ 
trated by these experiments. 

The effect of tarnish on the interior surface of the tube, or of an interior 
lining, upon the dynamic radiation from the surface is experimentally shown. 

The variation of the dvnamic radiation with the length of the radiating 
column is rendered manifest. The result has an important bearing on all 
experiments in which a thin stratum of moist air is the radiating source. 

A first comparison is instituted between the transmission of heat through 
vapours, and through the liquids from which they are derived. From this 
comparison the inference may be drawn that when a liquid is a powerful 
absorber, the vapour of that liquid is sure also to be a powerful absorber. The 
relation here revealed is more fully developed in Memoir YI. 


\ 


y. 

ON" THE ABSORPTION AND RADIATION OP HEAT 
BY GASEOUS AND LIQUID MATTER* 

INTRODUCTION. 

Tee Royal Society lias already done me the honour of publishing 
in the ‘ Philosophical Transactions 5 various memoirs on the 
relations of radiant heat to the gaseous form of matter. In the 
first of these f it was shown that for heat emanating from the 
blackened surface of a cube filled with boiling water, a class 
of bodies which had been previously regarded as equally, and 
indeed, as far as laboratory experiments went, perfectly* dia¬ 
thermic, exhibited vast differences both as regards radiation 
and absorption. At the common pressure of one atmosphere the 
absorptive energy of olefiant gas, for example, was found to be 
290 times that of air, while when lower pressures were employed 
the ratio was still greater. The reciprocity of absorption and 
radiation on the part of gases was also experimentally established 
in this first investigation. 

In the second inquiry J I employed a different and more 
powerful source of heat, my desire being to bring out with still 
greater decision the differences which revealed themselves in 
the first investigation. By carefully purifying the transparent 
elementary gases, and thus reducing their action upon radiant 
heat, the difference between them and the more strongly acting 
compound gases was greatly augmented. In this second inquiry, 
for example, olefiant gas, at a pressure of one atmos 2 )here, was 

* Received and read at the Royal Society June 18,1863. Philosophical Transactions, 
1864, p. 201 ; Philosophical Magazine , August 1864. 

t Philosophical Transactions, February 1861; and Philosophical Magazine, Sep¬ 
tember 1861. Memoir I. of this series. 

\ Philosophical Transactions, January 1862; and Philosophical Magazine, October 
1862. Memoir II. of this series. 


166 


THE ABSORPTION AND RADIATION OF HEAT • 


shown to possess 970 times the absorptive energy of atmospheric 
air, while it was shown to be probable that, when pressures of 
-Ajtli of an atmosphere were compared, the absorption of olefiant 
gas is nearly 6,000 times that of air. A column of ammoniacal 
gas, moreover, 3 feet long, was found sensibly impervious to the 
heat employed in the inquiry, while the vapours of many of the 
volatile liquids were proved to be still more opaque to radiant 
heat than even the most powerfully acting permanent gases. 
In this second investigation, the discovery of dynamic radiation 
and absorption is also announced and illustrated, and the action 
of odours and of ozone on radiant heat is made the subject of 
experiment. 

The third paper * of the series referred to was devoted to the 
examination of one particular vapour, which, on account of its 
universal diffusion, possesses an interest of its own—I mean, of 
course, the vapour of water. All the objections which had been 
urged against my results up to the time when the paper was 
written were here considered. I replied to each of them by 
definite experiments, removing them one by one ; and finally 
placing, as I believe, beyond the pale of reasonable doubt the 
action of the aqueous vapour of our atmosphere. In this third 
paper, moreover, the facts established by experiment were 
applied to the explanation of various atmospheric phenomena. 

§i. 

Further Experiments on the Poiver of Gaseous flatter over Radiant 
Heat—New Apparatus—Absorption by gaseous Strata of 
different thicknesses. 

In the present memoir an attempt is made to bring further 
into view both the power and the differences of power of gaseous 
bodies over radiant heat. Hitherto the gases and vapours 
operated on were introduced in succession into the same experi¬ 
mental tube, the heat being thus permitted to pass through the 
same thickness of different gases. The earlier part of the pre¬ 
sent inquiry is devoted to the examination of the transmission 
of radiant heat through different thicknesses of the same gaseous 
body. The brass tube with which my former experiments were 

* Philosophical Transactions, December 1852; and Philosophical Magazine, Julv 
1863. Memoir III. of this series. 


BY GASEOUS AND LIQUID MATTER. 


167 



conducted is composed of several pieces, which are screwed 
together when the tube is to be used as a whole; but they may 
be dismounted and used separately in lengths, varying from 
2*8 inches to 49*4 inches. I wished, however, to operate 
upon gaseous strata much thinner than the thinnest of 


these; and for this purpose a special apparatus was devised, 
and, with much time and trouble, rendered at length practically 
effective. 




























































































168 


THE ABSORPTION AND RADIATION OF HEAT 


The apparatus is sketched in fig. 13. C is the source of heat, 
consisting of a plate of copper against the back of which a 
steady sheet of flame is . caused to play. The copper plate 
forms one end of the chamber F (the 4 front chamber 5 of the 
former memoirs). This chamber, as in previous investiga¬ 
tions, passes through the vessel V, through which cold water 
constantly circulates, entering at the bottom and escaping at 
the top. The heat is thus prevented from passing by conduction 
from the source of heat C to the first plate of rock-salt S. * The 
plate S' closes the hollow cylinder A B, dividing it from the front 
chamber F, with which the cylinder A B is connected by suitable 
screws and washers. Within the cylinder A B moves a second 
one, 11, as an air-tight piston, closed at the bottom by the 
plate of rock-salt S'. The stuffing which renders the piston 
air-tight is seen in section at x and y. To make it perfect was 
the main difficulty of construction. The plate S' projects a 
little beyond the end of the cylinder 11; and can therefore be 
brought into flat contact with the other plate S. Fixed firmly 
to A B is a graduated strip of brass, while fixed to the piston is 
a second strip, the two strips forming a vernier, v v, By the 
pinion B, which works in a rack, shown above 11 in the figure, 
the two plates of salt may be brought near each other or 
separated, their exact distance apart being given by the vernier 
vv. P is the thermo-electric pile with its two conical reflec¬ 
tors; C' is the compensating cube, employed to neutralize the 
radiation from the source of heat C. H is an adjusting screen, 
by the motion of which the neutralization may be rendered per¬ 
fect, and the needle brought to zero under the influence of the 
two opposing radiations. The graduation of the vernier was so 
arranged as to permit of the employment of layers of gas varying 
from 0-01 to 2-8 inches in thickness. They were afterwards 
continued with the segments of the experimental tube, already 
referred to, and in this way layers of gas were examined which 
varied in thickness in the ratio of 1 : 4900. 

In my foimer experiments the chamber F was always kept 
exhausted, so that the rays of heat passed immediately from the 
source of heat through a vacuum; but in the present instance, 
fearing the strain upon the plate S, fearing also the possible 
intrusion of a small quantity of the gas under examination into 
the front chamber F, if the latter were kept exhausted, and 




BY GASEOUS AND LIQUID MATTER. 


169 


having proved that a length of 8 inches of dry air exerts no 
sensible action on the rays of heat, I had no scruple in filling 
the chamber F with dry air. Its absorption was nil , and it 
merely had the effect of lowering in an infinitesimal degree, by 
convection, the temperature of the source of heat. The two 
stopcocks c and c stand exactly opposite to the junction of the 
two plates of salt, S S', when they are in contact, and when they 
are drawn apart these cocks are in communication with the 
space between the plates. 

After many -trials to secure the best mode of experiment, the 
following one was adopted;—The holder containing the gas to 
be examined was connected by an india-rubber tube with the 
cock c , the other cock c being at the same time left open. The 
piston was then moved by the screw R until the requisite 
distance between the plates was obtained. This space being 
filled with dry air, the radiations on the two faces of the pile 
were equalized, and the needle brought to zero. The cock of 
the gas-holder was now opened, and by gentle water pressure 
the gas was forced first through a drying apparatus, and then into 
the space between the plates of salt. The air was quickly 
displaced, and a layer of the gas substituted for if. When the 
layer of gas possessed any sensible absorbing power, the equili¬ 
brium of the two sources of heat was destroyed; the source C' 
triumphed, and from the deflection due to its preponderance 
the exact proportion of heat intercepted by the gas could be 
calculated. 

When oxygen, hydrogen, or nitrogen was substituted for 
atmospheric air, no change in the position of the galvanometer- 
needle occurred; but when any one of the compound gases was 
allowed to occupy the space between the plates, a measurable 
deflection ensued. The plates of rock-salt were not so smooth, 
nor was their parallelism so perfect as entirely to exclude the gas 
*when they were in contact. Hence a stratum of gas sufficient, 
though but of filmy thickness, to effect a sensible absorption, 
could find its way between the plates even when they touched 
each other. On this account the first distance in the following 1 
tables was always really a little more than 0*01 of an inch. 


170 


THE ABSORPTION AND RADIATION OF HEAT 


Table I.— Carbonic Oxide. 


Thickness of 
gas 

Absorption in 
hundredths 
of the total 
radiation 

Thickness of 
gas 

Absorption in 
hundredths 
of the total 
radiation 

o-oi 

of 

an inch 

02 

0’4 of an inch 

3o 

0-02 


99 

0o 

0'5 „ 

3-8 

003 


99 

07 

0-6* 

4-0 

0-04 


99 

09 

1 

5*1 

0-06 


99 

1-4 

1-5 

6*1 . 

o-i 

03 


99 

11 

1-6 

3 

2 

6-8 


Table II. — Carbonic Acid. 


0-01 

of an inch 

0-86 1 

© 

A 

o 

•-+» 

an inch 

5-3 

0-02 

11 

1-2 

0-5 

11 

57 

0-03 

99 

1-5 

0-6 

11 

59 

0-04 

99 

1-9 

0-7 

11 

6 

005 

11 

21 

0-8 

11 

6-1 

0*06 

11 

23 

0-9 

11 

6-2 

o-i 

11 

33 

1 

11 

6-3 

0*2 

11 

41 

1-5 

11 

7 

0-3 

11 

4:8 

2 

11 

76 


Table III. — Nitrous Oxide. 


C’01 of an inch 

1-48 

0 4 of an inch 

10*20 

0-02 

233 

0o 

11 

0-03 

3-80 

0-6 

11-70 

0-04 

4 

0-8 

12-17 

0-05 „ 

4-20 

1 

12-80 

01 

6 

15 

14*20 

0-2 

7-77 

2 

15-7 


Table IY.— 

-Olefiant Gas. 


0’01 of an inch 

1-80 

0-5 of an inch 

23-30 

0-02 

3-08 

1 

26-33 

0-05 

5-37 

2 

32*80 

01 

914 




§ 2 . 

Effect of an Atmospheric Shell of Gas or Vapour two inches thick 

upon the Temperature of a Planet. 

We here find that a layer of olefiant gas only two inches in 
thickness intercepts nearly 33 per cent, of the radiation from 
our source of heat. Were our globe encircled by a shell of 











BY GASEOUS AND LIQUID MATTER. 


171 


olefiant gas of this thickness, the shell would offer a scarcely 
sensible obstacle to the passage of the solar rays earth¬ 
ward, but it would intercept, and in great part return, 33 
pei cent, of the terrestrial radiation. Under such a canopy, 
trifling as it may appear, the surface of the earth would be kept 
at a stifling temperature. The possible influence of an atmo¬ 
spheric envelope on the temperature of a planet is here most 
forcibly illustrated. 

The only vapour examined with this piston apparatus is that 
of sulphuric ether. Glass fragments were placed in a U-tube 
and wetted with the ether. Through the tube dry air was 
gently forced, whence it passed, vapour-laden, into the space 
between the rock-salt plates, S S / . The following table contains 
the results :—■ 


Table V .—Air saturated with the Vapour of Sulphuric Ether . 


Thickness of 

Absorption in 
hundredths 


Thickness of 

Absorption in 
hundredths 

vapour 

of the total 
radiation 


vapour 

of the total 
radiation 

O'05 of an inch 

2-07 

00 

© 

of an inch 

21-0 

01 

46 

1-5 

99 

346 

•» 

<M 

© 6 

8-7 

14-3 

2 

99 

351 


Comparing these results with those obtained with olefiant 
gas, we find for thicknesses of 0-05 of an inch and 2 inches 
respectively the following absorptions :— 


Olefiant gas 

Thickness of 0*05 . . 5‘37 

Thickness of 2 inches . 32 - 80 


Sulphuric ether 

Thickness of 0'05 . . 2‘07 

Thickness of 2 inches . 35-1 


Sulphuric-ether vapour, therefore, commences with an absorp¬ 
tion much lower than that of olefiant gas, and ends with a 
higher absorption. This is quite in accordance with the result 
established in my second memoir,* where it has been shown that 
while in a short tube the absorption effected by the sparsely 
scattered molecules of a vapour is far less than that of a gas at 
the pressure of an atmosphere, in a long tube the gas is excelled 
by the vapour. Still more impressively than that of olefiant gas, 
the deportment of sulphuric ether shows what mighty changes 
of climate might be brought about by the introduction into the 


* Philosophical Transactions , part i. 1862; and Philosophical Magazine, vol. xxiv. 
p. 343. Memoir II. of this series; § 14. 




172 


THE ABSORPTION AND RADIATION OF HEAT 


earth’s atmosphere of an almost infinitesimal amount of a 
powerful vapour. And if aqueous vapour can be shown to be 
thus powerful, the effect of its withdrawal from our atmosphere 
may be inferred. * 


§ 3. 

Hew Method of Experiment and its results—Division of Experi¬ 
mental Tube into two chambers—Transmission of Radiant Heat 

through Gases in one or both. 

The inquiry was extended to greater thicknesses of gas, by 
means of the composite brass experimental tube already referred 
to. The arrangement adopted, however, was peculiar, being 
expressly intended to check the experiments, which, under my 
supervision, were for the most part made by my assistants. 
The source of heat and the front chamber remained as usual, a 
plate of rock-salt dividing, as in my previous investigations, 
the front chamber from the experimental tube. The distant 
end of the tube was also stopped by a plate of salt; but, instead 
of permitting it to remain continuous from beginning to end, 
the experimental tube was divided, by a third plate of rock- 
salt, into two air-tight compartments. Thus the rays of heat 
from the source had to pass through three distinct chambers, 
and through three plates of salt. The first chamber was always 
kept filled with perfectly dry air. while either or both of the 
other chambers could be filled at pleasure with the gas or vapour 
to be examined. For the sake of convenience, I will call the 
compartment of the experimental tube nearest to the front 
chamber the first chamber, and the compartment nearest to 
the pile the second chamber, the term ‘ front chamber 5 being, 
as before, restricted to that nearest to the source of heat. The 
arrangement is sketched in outline in fi g;. 14. 

The entire length of the tube was 49*4 inches, and this 
length was maintained throughout the whole of the experiments. 
The only change consisted in the shifting of the plate of salt, 
S', which formed the partition between the first and second 
chambers. Commencing with a first chamber of 2*8 inches 
long, and a second chamber 46*4 inches long, the former was 
gradually augmented, and the latter equally diminished. The 
actual course of experiment was this:—The first and second 



BY GASEOUS AJST) LIQUID MATTER. 


173 


chambers being thoroughly cleansed and exhausted, the needle 
was brought to zero by the equalization of the radiations on 



the opposite faces of the pile. Into the first chamber the gas 
or vapour to be examined was then introduced, and its absorp- 






























































174 THE ABSORPTION AND RADIATION OF HEAT 

tion determined. This accomplished, the first chamber was 
cleansed, and the gas or vapour was introduced into the second 
chamber, its absorption there being also determined. Finally, 
both the chambers were filled and their joint absorption was 
determined. 

The combination here described enabled me to check the 
experiments, and also to trace the influence of the first 
chamber on the radiation. In it the heat was more or less 
sifted, the calorific beam entering the second chamber deprived 
of certain constituents which it possessed on its entrance into 
the first. On this account the quantity absorbed in the second 
chamber when the first is full of gas must always fall snort of 
the quantity absorbed when the first chamber is empty. From 
this it follows that the sum of the absorptions of the two 
chambers, taken separately, must always exceed the absorption 
of the tube taken as a whole. This may be briefly and con¬ 
veniently expressed by saying that the sum of the absorptions 
ought , on theoretic grounds, to exceed the absorption of the sum. 



Table 

YI. —Carbonic Oxide. 


Length 


Absorption per 100 


- - 

1st 

2nd 

r 

1st 

2nd 

Both 

Chamber Chamber 

Chamber 

Chamber 

Chambers 

28 

46-6 

6-8 

12-9 

12*9 

8 

41*4 

96 

12*2 

12*9 

12-2 

37-2 

10-7 

12-2 

12*9 

15-4 

34 

10.9 

12*2 

13*4 

17-8 

31-6 

11-1 

12 

13*3 

36*3 

13-1 

126 

10*3 

13*4 


Table 

YII.— Carbonic 

Acid. 


2-8 

46-6 

8-6 

13*8 

13*3 

8* 

41*4 

9-9 

127 

13*0 

12-2 

37-2 

11 

11*4 

13 

15*4 

340 

11-8 

12*1 

13*9 

238 

25*6 

11'7 

11*4 

13*1 

23-8 

25*6 

11*2 

11*2 

12*6 

23-8 

25*6 

10*4 

10*5 

12 

36-3 

13*1 

11*6 

10 

12*3 


Yarious causes have rendered these experiments exceedingly 
laborious. Could I have procured a sufficiently large quantity 
of gas in a single holder for an entire series of experiments, it 
would not have been difficult to obtain concurrent results, but 
the slight variations in quality of the same gas generated at 





BY GASEOUS AND LIQUID MATTER. 


I hf k 

to 

different times tell upon the results and render perfect uniformity 
extremely difficult of attainment. The approximate constancy 
of the numbers in the third column is, however, a guarantee 
that the determinations are not very wide of the truth. Irre¬ 
gularities, however, are revealed. Some remarkable ones occur 
in the case of carbonic acid, with the chambers 23*8 and 25*6 — 
the absorptions in the first chamber varying in this instance 
from 11*7 to 10*4, and in the second chamber from 1T4 to 10*5, 
and in both chambers from 13T to 12'0. The gas which gave 
the largest of these results was generated from marble and 
hydrochloric acid; the next was obtained from chalk and 
sulphuric acid, and the gas which gave the smallest result was 
obtained from bicarbonate of soda and sulphuric acid. The 
slight differences accompanying these different modes of gene¬ 
ration made themselves felt in the manner recorded in the 
table. 

Table YIII. — Nitrous Oxide. 


Length 

-A. 


Absorption per 100 


1st 

2nd 

( 

1st 

2nd 

Both 

Chamber 

Chamber 

Chamber 

Chamber 

Chambers 

28 

466 

161 

32-9 

33-9 

122 

37-2 

23-1 

30 

32 

15-4 

34 

23-6 

29-6 

32 

17-8 

31-6 

26-2 

296 

32-7 


The differences arising from different modes of generation 
are most strikingly illustrated by the powerful gases. Dr. 
Frankland, for example, was kind enough to superintend 
for me the making of a large holder of olefiant gas by the so- 
called c continuous process/ in which the vapour of alcohol is 
led through sulphuric acid diluted with its own volume of 
water. The following results were obtained:— 

Table IX .—Olefiant Gas. 


Length Absorption per 100 


r 

1st 

Chamber 

2nd ' 

Chamber 

1st 

Chamber 

2nd 

Chamber 

Both 

Chambers 

2 8 

46-6 

346 

66-1 

677 

8 

41-4 

44*2 

6o-3 

67o 

15-4 

34 

53-6 

62-3 

67 


The agreement of the absorption of both chambers, the sum 
of which was the constant quantity of 49*4 inches, must be 
regarded as satisfactory; and this is the general character of 







176 


THE ABSORPTION AND RADIATION OF HEAT 


the results as long as we adhere to gas generated in the 
same way. On the other hand, olefiant gas produced by mixing 
the liquid alcohol with sulphuric acid and applying heat to the 
mixture, gave the results recorded in the following table:— 


Table X .—Olefiant Gas. 


Length 



Absorption per 100 


1st 

2nd 

f 

1st 

2nd 

Both 

Chamber Chamber 

Chamber 

Chamber 

Chambers 

12*2 

37-2 

54-8 

70 

76*3 

15-4 

34 

59-1 

72-7 

77-1 

198 

29 6 

67-8 

70-4 

77 

23-8 

256 

69-2 

702 

77-6 

36-3 

13-1 

72*8 

60-3 

78-8 


Here the joint absorption of the two chambers is about 
10 per cent, higher than that of the gas generated under 
Dr. Frankland’s superintendence. 


§ 4 . 


Influence of 1 Sifting ’ by Gaseous Media. 

A few remarks on these results may be introduced here. In 
the case of carbonic oxide (Table VI.), we see that while a length 
of 2*8 inches of gas is competent, when acting alone, to inter¬ 
cept 6-8 per cent, of the radiant heat, the cutting off of this 
length from a.tube 49*4 inches long, or, what is the same, the 
addition of this length to a tube 46*6 inches long, makes no 
sensible change in its absorption. The second chamber absorbs 
as much as both. The same remark applies to carbonic acid, 
and it is also true within the limits of error for nitrous oxide 
and olefiant gas. Indeed it is only when 8 inches or more of 
the column have been cut away that the' difference begins to 
make itself felt. Thus, in carbonic oxide, the absorption of a 
length of 41*4 being 12*2, that of a chamber 49*4, or 8 inches 
longer, is only 12*9, making a difference of only 0*7 per cent., 
while the same thickness of 8 inches acting singly on the gas pro¬ 
duces an absorption of 9'6 per cent. So also with regard to 
carbonic acid; a tube 41*4 absorbing 12-7 per cent., a tube 
49*4 absorbs only 130 per cent., making a difference of only 
0*3 per cent. In the case of olefiant gas also (Table IX.), 
while a distance of 8 inches absorbs, acting singly, 44 per cent., 




BY GASEOUS AXB LIQUID MATTER. 177 

the addition of 8 inches to a tube already 41*4 inches long 
raises the absorption only from 65*3 to 67*5, or 2'2 per cent. 
The reason is plain. In a length of 41*4 the rays capable of* 
being absorbed by the gas are so much diminished, so few in 
fact remain to be attacked, that an additional 8 inches of gas 
produce a scarcely sensible effect. SimilaV considerations 
explain the fact that, while by augmenting the length of the 
first chamber from 2*8 inches to 15*4 inches we increase the 
absorption of olefiant gas nearly 20 per cent., the shortening of 
the second chamber by precisely th£ same amount effects a 
diminution of barely 4 per cent, of the absorption. All these 
results conspire to prove the heterogeneous character of the radia¬ 
tion from a source heated to about 250° G. 

The ‘ sum of the absorptions ’ and the ‘ absorption of the 
sum,’ placed side by side, exhibit the influence of sifting in an 
instructive manner. Tables VI., VII., VIII., IX., and X., thus 
treated, give the following comparative numbers:— 


Table XI. — Carbonic Oxide. 


Length of Chambers 

Sum of Absorptions 

Absorption of Sum 

2-8 


46-6 

19*7 

12-9 

8 


41-1 

21-8 

12-9 

12-2 


37*2 

22-9 

12-9 

lo’4 


34 

23*1 

13-4 

17-8 


31-6 / 

23-1 

13 3 

36*3 

• 

13-1 

22-9 

13-4 



Means 

. 22-3 

13-1 


Table XII. — Carbonic Acid. 


28 

46-6 

22-4 

13-3 

8 

41-4 

22-6 

13 

12-2 

37-2 

22-4 

13 

Id -4 

34 

23-9 

13-9 

23-8 

25-6 

23-1 

13-1 

363 

13-1 

21-6 

12 3 


Means . 

. 22-6 

131 


Table XIII.— 

-Nitrous Oxide. 


2-8 

46-6 

49 

339 

12-2 

37-2 

53-1 

32 

15-4 

34 

53-2 

32 

17-8 

31-6 

56-8 

32-7 


Means , 

. 52-8 

32-7 

12 









ITS THE ABSORPTION AND RADIATION OF HEAT 



Table XIV. 

—Olefiant Gas. 


Length of Chambers 

Sum of Absorptions 

Absorption of Sum 

2-8 

46-6 

1007 

67-7 

8 

414 

109-5 

675 

12-2 

37*2 

109-4 

65 

15-4 

34 

115-9 

67 


Means . 

. 1089 

668 


Table XV.- 

t 

• 

—Olefiant Gas. 


12-2 

37*2 

124-8 

76-3 

15-4 

34 

131-8 

77-1 

19-8 

296 

138-2 

77 

23-8 

256 

139-4 

il o 

363 

13-1 

133-1 

78-8 


Means . 

. 133-4 

77-3 


The conclusion that the sum of the absorptions is greater 
than the absorption of the sum is here amply verified. The 
tables also show the ratio of the sum of the absorptions to the 
absorption of the sum to be practically constant for all these 
gases. Dividing the first mean by the second in the respective 
cases, we have the following quotients :— 

Carbonic oxide . . . . . 170 

Carbonic acid.1*72 

Nitrous oxide. . . . . ,» 161 

Olefiant gas (mean of both) . . . 1-68 

The sum of the absorptions ought to be a maximum when 
the two chambers are of equal length. For, let them be unequal, 
one of them being in excess of half the length of the tubej 
and let us consider the action of this excess. Placed after the 
half-length, it receives the rays which have already traversed 
that half; placed after the shorter length, it receives the rays 
which have traversed the shorter length. In the former case, 
therefore, the 4 excess 9 will absorb less than in the latter, because 
the rays in the former case have been more thoroughly sifted 
before the heat reaches the excess. From this it is clear that, 
more is gained in the way of absorption by attaching the excess 
to the short length of the tube than to the half-length ; in other 
words, the sum of the absorptions, when the tube is divided 
into two equal parts, is a maximum. This reason is approxi- 




by gaseous and liquid matter. 


179 

mately verified by the experiments. As one length augments, 
and the other diminishes, we constantly approach the limit 
when the sum of the absorptions and the absorption of the sum 
are equal to each other. The effect of proximity to this limit 
is exhibited in the first experiment in each of the series ; where 
the lengths of the compartments are very uneqital, and the sum 
of the absorptions is, in general, a minimum. 


§ 5 . 


Application of Method to Vapours. 


After the absorption by the permanent gases had been in this 
way examined, I passed on to the examination of vapours. 
They were all used at a common pressure of 0*5 of an inch of 
mercury, or about -J^th of an atmosphere. The liquid which 
yielded the vapour was enclosed in the fiasks described in mv 
previous memoirs, and the pure vapour was allowed to enter the 
respective compartments of the experimental tube without the 
slightest ebullition. The following series of tables contains 
the results thus obtained :— 


Table XVI .—Bisulphide of Carbon. Pressure 0*5 of an inch. 


Length Absorption per 100 


1st 

Chamber 

2nd 

Chamber 

t - 

1st 

Chamber 

2nd 

Chamber 

Both ' 

Chambers 

28 

46-6 

3-6 

7-6 

7-6 

8 

414 

4-4 

7-3 

7-6 

15-4 

34 

57 

6 

75 

17 8 

31-6 

5-8 

6-4 

7-5 

238 

25*6 

67 

6 

7-8 

Table 

XVII. — Chloroform. 

Pressure 0*5 of an inch. 

2-8 

466 

5 5 

15-9 

16*3 

8 

41-4 

9-2 

15-6 

16*8 

12-2 

37-2 

ig-5 

148 

17-1 

15-4 

34 

• 

11-6 

141 

16-9 

23 8 

25-6 

15 

14 

18-4 

36-3 

131 

15*6 

10-9 

17-2 

Table 

XVIII. — Benzol. 

Pressure 0*5 of an 

inch. 

28 

466 

4 

20 

. 20-6 

8 

41*4 

8-4 

17*3 

20-4 

12-2 

37*2 

9-8 

165 

19 

178 

31-6 

11-9 

15-7 

20-1 

23-8 

25-6 

143 

15*1 

21-0 








ISO THE ABSORPTION AND RADIATION OF HEAT 


Table XIX.— Iodide of Ethyl. Pressure 0*5 of an inch. 


Length 

Absorption per 100 

-- 


1st 

Chamber 


2nd 

Chamber 

r~ - 

1st 

Chamber 

2nd • Both 

Chamber Chambers 

2-8 


46-6 

7-1 

23-5 

25-4 

8 


. 41*4 

91 

21*1 

23-3 

12-2 


37-2 

12-8 

20o 

25*2 

15-4 


34 

14-6 

20-8 

25-2 

17 8 


31-6 

15-8 

20 

25 o 

Table 

XX. — Alcohol. 

Pressure 

0*5 of an inch. 

28 


466 

11-7 

46-1 

46-1 

8 


41-4 

18-5 

43-6 

47 

12-2 


372 

26 

44-1 

475 

15-4 


34 

32-1 

41-1 

47 

178 


31-6 

324 

40 

47-6 

Table 

XXI. — Alcohol. 

Pressure 

0T of an inch. 

8 


41-4 

8 

22-2 

249 

15-4 


34 

12-1 

20 

247 

17-8 


31-6 

13-1 

19 7 

257 

238 


256 

14-8 

18-4 

252 

363 


13-1 

19-1 

13-8 

257 

'able XXII.— Sulphuric Ether. Pressure 0-5 of 

an inch 

2-8 


46-6 

14 8 

50 

51-6 

8 


41-4 

239 

51 

539 

12-2 


372 

30-9 

488 

536 

15-4 


34 

34 

47'8 

531 

Table 

XXHI. — Acetic Ether. Pressure 0*5 of an inch. 

28 


46-6 

17 

60*2 

62-9 

8 


41-4 

307 

58*1 

64-6 

122 


37-2 

41-6 

55-1 

64-2 

15-4 


34 

44-4 

555 

62-4 

23-8 


25-6 

50-9 

527 

647 

363 


131 . 

58-1 

42-6 

64-8 

Table 

• 

XXIV. — Formic Ether. Pressure 0*5 of an inch. 

2-8 


466 

17-4 

63 

64-4 

8 


41-4 

33-3 

59-1 

63-4 

17-8 


31-6 

40 

484 

60-3 

•23-8 

• 

256 

456 

47-2 

60-2 


In the following tables the sum of the absorptions is com 
pared with the absorption of the sum in the case of vapours. 






BY GASEOUS AND LIQUID MATTER. 


181 


Table XXV. Bisulphide of Carbon. Pressure 0*5 of an inch. 


Length of Chambers Sum of 

Absorptions 

Absorption of Sum 

28 

46-6 

11-2 

7-6 

8 

4] *4 

11-7 

7-6 

15-4 

34 

11-7 

7-5 

* 17-8 

21-G 

12-2 

7-5 

23-8 

25-6 

127 

7-8 


Means . 

11-9 

7-6 

Table XXYI. 

— Chloroform. 

Pressure 

• 

0*5 of an inch. 

2-8 

46*6 

21-4 

16-3 

8 

41-4 

24-8 

16-8 

122 

37-2 

25 3 

17-1 

15-4 

34 

25-2 

16-9 

23-8 

256 

29 

18-4 

36-3 

33*1 

26-5 

172 


Means . 

25-36 

17-1 


Table 

XXVII. — Benzol. 

Pressure 0* 

5 of an inch. 

2-8 

466 

24 

20-6 

8 

41-4 

257 

20-4 

12-2 

37'2 

26-3 

19 

17-8 

31-6 

27-6 

20-1 

23-8 

256 

29-4 

21 


Means . 

. 26-6 

20-2 


Table XXVIII.- 

-Iodide of Ethyl 

. Pressure 

0*5 of an inch 

28 

46-6 

30-6 

25'4 

8 

41-4 

30-2 

23-3 

12-2 

372 

33-3 

25-2 

15-4 

34 

35-4 

252 

17 8 

31-6 

358 

252 


Means . 

33-1 

24-9 


Table XXIX. — Alcohol. 

Pressure 0*5 of 

an inch. 

28 46-6 

578 

46*1 

8 41-4 

62-1 

47 

12-2 37-2 

70*1 

475 

15-4 34 

73-2 

47 

17-8 31-6 

72-4 

47*6 

• Means . 

. 67-1 

47 










182 


THE ABSOKPTION AXD RADIATION OF HEAT 


Table XXX.— Alcohol. Pressure OT of an inch. 


Length of Chambers 

Sum of Absorptions 

Absorption of Sum 

8 

41-4 

30-2 

24-9 

154 

34 

321 

247 

17*8 

31-6 

328 

25-7 

23-8 

25-6 

332 

25-2 • 

36*3 

13-1 

32-9 

251 


Means 

. 322 

251 


Table 'XXXI.— Sulphuric Ether. Pressure 0-5 of an inch. 


2*8 

46-6 

648 

516 

8 

41-4 

749 

53-9 

12-2 

37-2 

79-7 

53-6 

154 

34 

81-8 

53-1 


Means . 

. 753 

53-05 


Table XXXII.— Formic Ether. Pressure 0*5 of an inch. 


2-8 

46-6 

80-4 

64-4 

8 

41-4 

82-4 

63-4 

17-8 

31-6 

88-4 

603 

23-8 

25-6* 

92-8 

60-2 


Means . 

86 

62-07 

XXXIII.- 

-Acetic Ether. 

Pressure 0*5 of ar 

2-8 

46-6 

77-2 

62-9 

8 

41-4 

88-8 

64-6 

12-2 

37*2 

96 7 

64*2 

15-4 

34 

99-9 

624 

23-8 

25-6 

103-6 

64-7 

36-3 

13-1 

100-7 

64-8 


Means . 

94-5 

63-9 


*\M 


In the case of vapours, the difference between the sum of the 
absorptions and the absorption of the sum is, in general, less 
than in the case of gases. This resolves itself into the proposi¬ 
tion that for equal lengths, within the limits of these experi¬ 
ments, the sifting power of the gas is greater than that of the 
vapour. The reason of this is that the vapours are examined 
at a pressure of an atmosphere and the vapours at a pressure 
of -g^th of an atmosphere. Thus, as before proved,* no matter 

l 

* Section 14, Memoir II. 









BY GASEOUS AND LIQUID MATTER. 


183 


liow powerful tlie individual molecules may be, their distance 
asunder renders a thin layer of them a comparatively open 
sieve. 



New Experiments on Dynamic Radiation.—Radiation of Dynami¬ 
cally heated Gas through the same Gas , or through other Gases . 

The entrance of a gas into an exhausted vessel is known to 
be accompanied by the generation of heat; and the gas thus 
warmed, if a radiator, will emit the heat generated. Con¬ 
versely, on exhausting a vessel containing any gas, the gas is 
chilled, and thus an external body, which prior to the act of 
exhaustion possesses the same temperature as the gas, becomes, 
on the first stroke of the pump, a warm body with reference to 
the gas remaining in the vessel. If the body be separated from 
the cooled gas by a diathermic partition, it will radiate into the 
gas and be more or less chilled. It was shown in my second 
memoir that this dynamic warming and chilling of a gas or 
vapour furnished a practical means of determining, without 
any source of heat external to the gaseous body itself, both its 
radiative and absorptive energy, the terms dynamic radiation 
and dynamic absorption being then for the first time introduced 
to express this newly-discovered action. 

During the last half-year a considerable number of experi¬ 
ments have been made in illustration of the manner in 
which dynamic radiation may be applied in researches on 
radiant beat. A few of these I will here describe. The 
source of heat was abolished; one end, S,, (fig. 15) of the 
experimental tube was stopped by a plate of polished metal, the 
other, S", by a transparent plate of rock-salt, while the space 
between the ends was divided into two compartments by a 
second plate of salt, S'. The thermo-electric pile, P, occu¬ 
pied its usual position at the end of the tube, the compensating 
cube, however, being abandoned. For the sake of convenient 
reference, I will call the compartment of the tube most distant 
from the pile the first chamber, and that adjacent to the pile 
the second chamber. 

Both chambers being exhausted and the needle at zero, the 
gas was allowed to enter the first through a gauge-cock which 









184 


THE ABSORPTION AND RADIATION OF HEAT 


made its time of entry 40 seconds ; tlie second chamber at the 
same time being preserved a vacuum. The gas on entering 
was dynamically heated, and radiated its heat to the pile 
through the vacuous second chamber. The needle moved, and 
the limit of its excursion was noted. The first chamber was 
then exhausted and carefully cleansed with dry air. The 
second chamber was filled with the same gas, not yet with a 
view to its dynamic radiation, but to examine its effect upon 
the heat radiated from the first chamber. The needle being 
at zero, the gas was again permitted to enter the first chamber 


Fig. 15. 



pile, while in the second it had to pass through a column of 
the same gas as that from which it emanated. The reciprocity 
of radiation and absorption could thus be illustrated in a novel 
and interesting manner. In this way, in fact, the absorption 
exerted by any gas not only on heat radiated from the same 
gas, but from any other gas, may be determined. Finally, the 
apparatus being cleansed and the needle at zero, the gas w T as 
permitted to enter the second chamber, and its dynamic radia¬ 
tion from this chamber determined. The intermediate plate 
of salt, S', was shifted, as in the former experiments, so as to 
alter the lengths of the two chambers, the sum of both lengths 
remaining constant as before. 

In the following tables the three columns bracketed under 
the head of ‘ Deflection,’ contain the arcs through which the 
needle moved in the three cases: (1) when the radiation 
































BY GASEOUS AND LIQUID MATTER. . 185 

from the gas in the first chamber passed through the empty 
second chamber; (2) when the radiation from the first chamber 
passed through the occupied second chamber; and (3) when 
the radiation proceeded from the second chamber. 


Dynamic Radiation of Gases. 
Table XXXIV. — Carbonic Oxide. 

Deflection 



Length 

t - 

A- 


< - 

-*-^ 

By 1st 

By 1st 


1st 

2nd 

Chamber, 

Chamber, 

By 2nd 

Chamber Chamber 

2nd Chamber 

G as in 2nd 

Chamber 



empty 

Chamber 




O 

O 

O 

2-8 

46-G 

1 

0 

28 

V54 

34 

3 8 

2-1 

24-4 

363 

131 

137 

63 

16-6 


Table 

XXXV .^-Carbonic Acid. 


2-8 

466 

• 

l 

0 

336 

15-4 

34 

37 

1 25 

233 

363 

13-1 

16-8' 

6-6 

17-5 


Table 

XXXVI.— Nitrous Oxide. 


28 

46-6 

l 

02 

445 

154 

34 

4-3 

1-2 

317 

363 

131 

19-5 

62 

22 


Table 

XXXVII.— Olefiant Gas. 


154 

34 

11-9 

1 

63 

23-8 

256 

228 

3 


36*3 

131 

59 

10-4 

65 


The gases, it will be observed, exhibit a gradually increasing 
power of dynamic radiation from carbonic oxide up to olefiant 
gas. This is most clearly illustrated by reference to the results 
obtained in the respective cases with the first length of the 
second chamber. They are as follows :— 


Carbonic oxide 
Carbonic acid . 
Nitrous oxide . 
Olefiant gas . 


. 28 
.• 336 
. 445 
. 68 


Its proximity to the pile, and.the fact of its having to cross 
but one plate of salt, make the radiation from the second 
chamber much greater than that of the first. 





186 THE ABSORPTION AND RADIATION OF HEAT 

All the tables show that as the length of the chamber in¬ 
creases, the dynamic radiation of the gas contained in it in¬ 
creases ; and as the length diminishes, the radiation diminishes. 
They also show how powerful!}" the gas in the second chamber 
acts upon the radiation from the first. With carbonic oxide, 
the introduction of a column of gas 13T inches long reduces 
the deflection from 13*7° to 6*3°, or more than 50 per cent. In 
the other cases the reduction is still greater. With carbonic 
acid it is reduced from 16*8 to 6*6; with nitrous oxide it is 
reduced from 19’5 to 6*2. Nor is this residual deflection, 6*2°, 
entirely due to the transparency of the gas, to heat emitted by 
the gas. No matter how well polished the experimental tube 
may be, there is always a certain radiation from its 1 interior 
surface when the gas enters it. With perfectly dry air this 
radiation amounts to 8 or 9 'degrees. Thus the radiation is 
composite, in part emanating from the molecules in the first 
chamber, and in part emanating from the surface of the tube. 
To these latter, the gas in the second chamber would be much 
more permeable than to the former; and to these latter, I 
believe, the residual deflection of 6 degrees, or thereabouts, is 
mainly due. That this number turns up so often, although the 
radiations from the various gases differ so considerably, is in 
harmony with the supposition just made. In the case of car¬ 
bonic oxide, for example, the deflection is reduced from 13’7° 
to 6*3°, while in the case of nitrous oxide it is reduced from 
19-5° to 6*2°; in the case of olefiant gas it is reduced from 
59° to 10*4°, while in other experiments (not here recorded) the 
deflection by olefiant gas was reduced from 44° to 6°. 

§ 7. 

Influence of Tarnish , or of a Lining on the Interior Surface of 

Experimental Tube.—Dynamic Radiation from the Surface. 

\ 

As may be expected, this radiation from the interior surface 
augments with the tarnish of the surface, but the extent to 
which it may be increased is hardly sufficiently known. Indeed 
the gravest errors are possible in experiments of this nature if the 
influence of the interior be overlooked or misunderstood. An ex¬ 
periment or two will illustrate this more forcibly than any words 
of mine. 


BY GASEOUS AND LIQUID MATTER. 


187 


A brass tube 3 feet long, and very slightly tarnished within, 
was used for dynamic radiation. Dry air on entering the tube 
produced a deflection of 12 degrees. The tube was then polished 
within, and the experiment repeated ; the action of dry air was 
instantly reduced to 7‘5 degrees. 

The rock-salt plate at the end of the tube was then removed, 
and a lining of black paper 2 feet long was introduced. 
The tube was again closed, and the experiment of allowing 
dry air to enter it repeated. The deflections observed in three 
successive experiments were 

80°, 81°, 80°. 

This result might be obtained as long as the lining continued 
within the tube. 

The plate of rock-salt was again removed, and the length of 
the lining was reduced to a foot; the dynamic radiation on the 
entrance of dry air in three successive experiments gave the 
deflections 

76°, 74°, 75°. 


The plate was again removed and the lining reduced to 3 
inches; the deflections obtained in two successive experiments 
were 

66°, 65°. 

Finally, the lining was reduced to a ring only 1J inch in 
width; the dynamic radiation from this small surface gave in 
two successive trials the deflections 

5 6°, 56'5°. 

The lining was then entirely removed, and the deflection in¬ 
stantly fell to 

7-5°. 

A coating of lampblack within the tube produced the same 
effect as the paper lining; common writing-paper was almost 
equally effective; a coating of varnish also produced large 
deflections, and the mere oxidation of the interior surface of 
the tube is also very effective. 

In the above experiments the lining was first heated, and it 
then radiated its heat through a thick plate of rock-salt against 
the pile. The effect was enfeebled by distance, by reflexion 






188 


THE ABSORPTION AND RADIATION OF HEAT 


from tlie surfaces of the salt, and by partial absorption. Still 
we see that the radiation thus weakened was competent to 
drive the needle almost through the quadrant of a circle. If, 
instead of being thus detached, the face of the pile itself had 
formed part of the interior surface of the experimental tube, 
the deflections would, of course, be far greater than the 
highest of those here recorded. I do not entertain a doubt 
that, by the dynamic heating of the surface of the pile, the 
needle of my galvanometer could be caused to whirl through 
an arc of 1000 degrees. Assuredly an arrangement subject to 
disturbances, or masking disturbances, such as these cannot be 
suitable in experiments in which the most refined delicacy is 
absolutely necessary. 


§ 8 . 

Radiation of dynamically heated Vapour through the same Vapour 
and through a Vacuum.—Influence of Length of Radiating 
Column.—Different Effects of Length on Gases and Vapours. 

Experiments similar to those recorded in § 6 were also made 
with vapours. Both chambers of the experimental tube being 
exhausted, the vapour was permitted to enter the first, and dry 
air to follow it. The air thus dynamically heated warmed the 
vapour, and the vapour radiated its heat against the pile. As 
in the case of gases, the heat passed in the first experiment 
through a vacuous second chamber, in the second experiment 
through the same chamber when it contained 0*5 of an inch of 
the same vapour as that from which the rays issued, while a 
third experiment was made to determine the dynamic radiation 
from the second chamber. The following tables contain the 
results: — 


Dynamic Radiation of Vapours. 

Table XXXVIII. —Bisulphide of Carbon. Pressure 0*5 of an inch. 


Length 
r~ - A - 

By 1st 

Deflection 

By 1 at 


1st 

‘2nd 

Chamber, 

Chamber, 

Bv 2nd 

Chamber 

Chamber 

2nd Chamber 
empty 

Vapour in 2nd 
Chamber 

Chamber 

154 

34 

O 

. 2-4 

O 

1-6 

O 

14-2 

36-3 

13-1 

975 

5 5 

9 






BY GASEOUS AND LIQUID MATTER. 


189 


Table XXXIX.— Benzol. Pressure 0*5 of an inch. 


Deflection 

Length , -*- 


, - 

A 

By 1st 

By 1st 


1st 

2nd 

Chamber, 

Chamber, 

By 2nd 

Chamber 

Chamber 

2nd Chamber 

Vapour in 2nd 

Chamber 



empty 

Chamber 




O 

0 

O 

154 

34 

3 

1-1 

34 

36-3 

131 

21*6 

11*9 

15-1 


Table XL .—Iodide of Ethyl. Pressure 0*5 of an inch. 

15-4 34 3*4 2'7 38 8 

36-3 13-1 25-4 13 8 19 


Table XLI.— Chloroform. 

V 

154 34 

363 131 

Table XLII.— Alcohol. 

15-4 34 

363 13-1 


Pressure 0*5 of an inch. 


4-5 2-1 41 

22-3 10 19 

Pressure 0*5 of an inch. 

4-9 2 53-8 

33-8 169 349 


Table XLIII.— Alcohol. Pressure 0T of an inch. 


15-4 

36*3 


34 

13*1 


2 

21-8 


1*3 

16-2 


35-7 

11*5 


Table XLIY .—Boracic Ether. Pressure 0T of an inch. 

15*4 34 6 3 2-1 61 

36-3 13*1 29-1 15-7 31*6 


Table XLY .—Formic Ether. Pressure 0*5 of an inch. 

15*4 34 6-3 2-5 68. 

36-3 13*1 46 23*8 41 


Table XLYI. —Sulphuric Ether. Pressure 0*5 of an inch. 

15-4 34 5 6 2 5 68 

363 131 453 22-4 36*5 


Table XLYII .—Acetic Ether. Pressure 0'5 of an inch. 

15-4 34 5-7 1 73-9 

36-3 13*1 49-1 22 41 





190 


THE ABSORPTION AND RADIATION OF HEAT 


Collecting the radiations from tlie second chamber for the 

o 

lengths 34 inches and 13*1 inches into a single table, we see at 
a glance how the radiation is affected by the length. 


Table XLVIII. 





Dynamic Radiation of various 




Vapours at 0 - 5-inch pressure 




and a common thickness of 




( : 

— N 




34 inches 

13*1 inches 




o 

o 

Bisulphide of carbon 



. 14-2 

9 

Benzol .... 



. 34 

15*1 

Iodide of ethyl 



. 38-8 

19 

Chloroform . 



. 41 

19 

Alcohol .... 



. . 53-8 

34-9 

Sulphuric ether 



. 68 

36*5 

Formic ether . 



. 68 

41 

Acetic ether . . . 



. 73*9 

41 




At a pressure of 0’1 of an inch 

Alcohol .... 



. 35-7 

11*5 

Boracic ether . 

• 

• 

. 61 

31-6 


The extraordinary energy of boracic ether as a radiant may 
be inferred from the last experiment. Although attenuated to 
-jA-Q-th of an atmosphere, its thinly scattered molecules are able 
to urge the needle through an arc of 61 degrees, and this merely 
by the warmth generated on the entrance of dry air into a 
vacuum. 

Arranging the gases in the same manner, we have the follow¬ 
ing results:— 

Table XLIX. 


Dynamic Radiation of Gases 
at 1 atm. pressure, and a 
common thickness of 


Carbonic oxide 





t - 

34 inches 

o 

13’1 inches 

o 

• 


• • 

• 

. 24-4 

16-6 

Carbonic acid 

• 


• • 

• 

. 23*3 

17-5 

Nitrous oxide 

• 


• • 

• 

. 317 

22 

Olefiant gas . 

• 


• • 

• 

. 68 

65 


As remarked in Memoir II. § 14, a greater length is avail¬ 
able for radiation in the case of the vapour than in the case of 
the gas, because the radiation from the hinder portion of the 
column of vapour is less interfered with by the molecules in 
front. By shortening the column therefore we diminish, and 












BY GASEOUS AXD LIQUID MATTER. 


191 


by lengthening it we promote, the radiation from the vapour 
more than that from the gas. Thus while a shortening of the 
column from 34 inches to 13T causes a fall in the case of car¬ 
bonic oxide only from 23*3° to 17*5°, the same amount of short¬ 
ening causes benzol vapour to fall from 34° to 15T°, a much 
greater diminution. So also as regards olefiant gas, a shorten¬ 
ing of the* radiating column from 34 inches to 13T inches causes 
a fall in the deflection only from 68° to 65° ; the same diminution 
produces with sulphuric ether a fall from 68° to 36*5°; and with 
acetic ether from 73*9° to 41°. In the 34-inch long column, 
moreover, acetic ether beats olefiant gas, but in the 13-inch 
column the gas beats the vapour. 

A series of experiments of this nature executed last autumn, 
though not free from irregularities, is nevertheless worth 
recording here. A brass experimental tube, slightly tarnished 
within, 49*4 inches long, and divided into two chambers, each 
24*7 inches long, was employed, and with the following re¬ 
sults :— 


Table L .—Dynamic Radiation of Vapours. 

Deflection 

t - A - 

By 1st By 1st 


Bisulphide of carbon . 


Chamber, 
2nd Chamber 
empty 

o 

8-2 

Chamber, 
Vapour in 2nd 
Chamber 

o 

5-8 

By 2nd 
Chamber 

o 

21-2 

Benzol 


. 20 % 

124 

45-9 

Chloroform 


. 24-3 

109 

55*2 

Iodide of ethyl . 


. 275 

147 

55-3 

Alcohol 


. 42-7 

22-3 

69 

Sulphuric ether . 


. 46-3 

21-7 

80-5 

Formic ether 


. 47-5 

19-8 

79-5 

Propionate of ethyl . 


. 49-8 

25 

82*3 

Acetic ether 


. 53-3 

30 

82*1 


§ 9. 

First Comparison of the Actions of Liquids and their Vapours 

upon Radiant Heat. 

To ascertain whether the action of these vapours bears any 
significant relation to that of the liquids from which they are 
derived, the transmission of radiant heat through those liquids 
was examined. The naked flame of an oil lamp was used as a 







192 


THE ABSORPTION AND RADIATION OF HEAT 


source, and the liquids were enclosed in rock-salt cells. Thus 
the total radiation from the lamp, with the exception of the 
minute fraction absorbed by the rock-salt, was brought to bear 
upon the liquid. 

In the following table the liquids are arranged in the order 
of their powers of transmission :— 

Table LI. 

Transmission in 


Name of Liquid H undredths of 

the Radiation 

Bisulphide of carbon.83 

Bisulphide of carbon saturated with sulphur . . . .82 

Bisulphide of carbon saturated with iodine . . . .81 

Bromine . . . . . . . • • .77 

Chloroform.73 

Iodide of methyl.69 

Benzol ........... 60 

Iodide of ethyl . .57 

Amylene .......... 50 

Sulphuric ether . . ..41 

Acetic ether. . . .34 

Formic ether.......... 33 

Alcohol.30 

Water saturated with rock-salt.26 


These results are but approximate, and the source of heat 
has been changed, still it is impossible to regard them without 
feeling how purely the act of absorption is a molecular act, and 
that when a liquid is a powerful absorber the vapour of that liquid 
is sure also to be a powerful absorber. 

To experiment with water, it was necessary to saturate it with 
the salt of which the cell was formed, but the absorptive energy 
is due solely to the water. We might infer from this alone, 
were no experiments made on the aqueous vapour of the atmo¬ 
sphere, that that vapour must exert a powerful action upon 
terrestrial radiation. In fact, in all the statements that I have 
hitherto made its action has been underrated. 

The deportment of the elements sulphur and iodine, dissolved 
in bisulphide of carbon, is in striking harmony with the other 
results which these researches have made known regarding the 
action of elementary bodies. The saturation of the bisulphide 
by sulphur scarcely affects the transmission, while a quantity 
of iodine sufficient to convert the liquid from one of perfect 









BY GASEOUS AND LIQUID MATTER. 


193 


transparency to one of almost perfect opacity to light, produces 
a diminution of only 2 per cent, of the radiation. This shows 
that the heat really used in these experiments consists almost 
wholly of the obscure rays of the lamp. The deportment of 
bromine is also very instructive. The liquid is very dense, and 
so opaque as to cut off the luminous rays of the lamp; still it 
transmits 77 per cent, of the total radiation. It stands in point 
of diathermancy above every compound liquid in the list, ex¬ 
cept bisulphide of carbon. This latter substance is the rock-salt 
of liquids. 

It is worth remarking that the obscure rays of a luminous 
source have a much greater power of penetration in the case of the 
liquids here examined than the rays from an obscure source , how¬ 
ever close to incandescence. 

Before a strict comparison can be made between vapours and 
liquids, they must both be examined by heat of the same quality , 
and arrangements have been already made with which I hope 
to obtain more complete and accurate results than those above 
recorded. 


13 




f 








YI. 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 



ANALYSIS OF MEMOIR VI. 


The comparison of vapours and liquids, roughly illustrated towards the conclu¬ 
sion of the last inquiry, prompted a more accurate examination of this important 
question. The present memoir begins with the description of the necessary 
apparatus. A source of heat of perfectly definite quality is obtained by sending 
through a platinum spiral a voltaic current of a constant strength. Rock-salt 
cells are also devised wherein the liquids are enclosed, and the action upon 
radiant heat of eleven different liquids in five different thicknesses is deter- 
mined. 

The action of the vapours of those liquids upon radiant heat of the same 
quality is then determined. 

In each series of experiments the different liquids are first compared at a 
common thickness, and the vapours at a common pressure. A striking general 
agreement was established, the order of absorption in the two cases being almost 
the same. 

Fresh experiments are then executed in which the liquids and their vapours • 
are rendered proportional to each other in quantity. Thus compared, the order 
of absorption in the two cases is proved to be identical. 

It is hence inferred that the position of any vapour as an absorber of radiant 
heat is determined by the position of its liquid. No experiment has ever been 
made to shake the validity of this inference. From the deportment of water, 
therefore, the deportment of its vapour may be inferred, and the influence of 
this agent in the phenomena of meteorology anticipated and understood. 

The special bearing of chemical constitution on absorption is made the sub¬ 
ject of a brief section. 

In the spring of 1862 coloured elementary liquids, embracing bromine and 
dissolved iodine, were examined and found exceedingly pervious to the obscure 
heat-ravs. In the present investigation, special experiments are made with a 
view of accurately determining the diathermancy of dissolved iodine. It is 
shown to be practically perfect for obscure rays. 

The influence of the temperature of the source upon the penetrative power of 
heat-rays has been the subject of frequent discussion among philosophers. 
The arrangement adopted in this inquiry, which permits of our varying the 
temperature at will, while retaining throughout the same vibrating atoms , enables 
us to put this question in a clearer light than usual. The platinum spiral is 
employed at a barely visible heat, at a bright red heat, at a white heat, and also 
at a heat close to fusion; the variations of transmissive power consequent upon 
these changes of temperature being recorded. 

Experiments are described which show some very singular shiftings of the 
diathermic position of vapours consequent on varying the source of heat. 
Starting with a low heat, formic ether proves a more powerful absorbent than 
sulphuric ether; but as the temperature augments they become equal, and at a 
white heat their positions are reversed. When the source is a blackened cube 


ANALYSIS OF MEMOIR VI. 107 

« 

of boiling water, formic ether also decidedly excels sulphuric in absorbent 
power. 

Chloroform, at all temperatures of the platinum source, shows itself a 
feebler absorber than iodide of methyl; but when the source is a cube coated 
with lampblack it becomes the more powerful absorbent. The differences in 
the quality of the emission from different solid bodies of the same temperature 
are thus strikingly illustrated, various peculiarities of molecular vibration being 
at the same time brought into view. 

The source is next changed to flames of different kinds; luminous gas-jets, 
the pale blue flame of Bunsens burner, hydrogen flames, and carbonic-oxide 
flames being successively invoked. The radiation from the incandescent carbon 
particles of the gas-flame is contrasted with that from white-hot platinum. 
Other inversions of diathermic position are made manifest. When a luminous 
gas-jet is the source, bisulphide of carbon is decidedly more diathermic than 
chloroform ; when a Bunsen’s flame is the source, or even when the gas-flame 
is much reduced in size and brilliancy, chloroform is decidedly more diathermic 
than bisulphide of carbon. The removal of the white-hot carbon particles from 
the gas-flame more than doubles the relative diathermancy of the chloroform. 
When, moreover, a carbonic-oxide flame is the source, sulphuric ether excels 
formic ether in absorption ; but when the source is a hydrogen flame the formic 
ether excels the sulphuric. 

In every case here mentioned the deportment of the vapour was compared with 
that of its liquid , and it was found that every change in the position of the one was 
accompanied by a corresponding change in the position of the other. 

The opacity of a gas or vapour to radiation from its own molecules has been 
frequently adverted to in these memoirs. To determine whether the radiation 
from a hydrogen flame, where we have aqueous vapour in a state of incandes¬ 
cence, is intercepted with peculiar energy by the cold aqueous vapour of the 
atmosphere, special experiments were instituted. The result justified the trial; 
for on a day when less than 6 per cent, of the heat from a red-hot platinum 
wire was absorbed by moist air, as nearly as possible three times this quantity 
. was absorbed of the radiation from a hydrogen flame. The mere plunging a 
platinum spiral into the hydrogen fame doubled the transmission through humid 
air. 

The radiation from a carbonic-oxide flame through carbonic acid is a case of 
still more striking interest. Among the compound gases, carbonic acid is one 
of the feeblest absorbers. But when it receives the rays from its own sub¬ 
stance heated to incandescence, as in the carbonic-oxide flame, its opacity is 
astounding. It far transcends olefiant gas, which for all ordinary radiations 
far transcends it. So energetic is the action that the carbonic acid expired 
from the lungs and freed of its moisture intercepts fully 50 per cent, of the radia¬ 
tion from the carbonic-oxide fame. The method of experiment may be turned 
to account as a powerful and delicate test of the amount of the exhaled carbonic 
acid. 

It is then remarked that the deportment of aqueous vapour towards the 
hydrogen flame and of carbonic acid towards the carbonic-oxide flame, prove the 
molecules of the two flames, the one having a temperature of 3259° C., and the 
other a temperature of 3042° 0., to vibrate in synchronism with the molecules 
of cold aqueous vapour and cold carbonic acid. 

Throughout the memoir this conception of discord and accord, between the 


198 


ANALYSIS OF MEMOIR VI. 


periods of incident waves and of tlie molecules on which they are incident, is 
kept constantly in view; and by its aid a number of phenomena which have 
hitherto withstood explanation are reduced to order and clearness. Some very 
interesting results, published by Melloni and Knoblauch, are thus explained. 

The diathermancy of bodies opaque to light is still further illustrated. 
Forty-one per cent, of the radiation from a hydrogen flame is shown to be 
transmitted by opaque soot; and 99 per ceut. by opaque iodine. 

The radiation of flames through liquids is made the subject of varied experi¬ 
ment ; the change, and meaning of the change, caused by the introduction of 
solid bodies into flames, being demonstrated. 

The following reasoning on the facts established in fliis memoir leads up to 
a conclusion of theoretic importance. From the singular opacity of water to 
the radiation of a hydrogen flame, the synchronism of the molecules of the liquid 
with those of the flame is inferred. 

But from its opacity to the ultra-red undulations the synchronism of water 
with the longer waves of the spectrum may, also be inferred; hence the emission 
from the hydrogen flame, which synchronises with water, must be mainly ultra- 
red. 

It therefore follows that when a platinum wire is plunged into hydrogen and 
caused to glow with a white heat; or when the oxyhydrogen jet raises lime 
to dazzling incandescence, the light is produced by converting the long unvisual 
waves of the hydrogen and oxyhydrogen flames into shorter and visual ones. 

Whether we have a vibrating atom of ponderable matter or a vibrating 
particle of the luminiferous aether, the principle, it is contended, is the same. 
And it is inferred that obscure radiant heat , if it could be rendered sufficiently 
intense, would, like the hydrogen flame, be competent to raise bodies to in¬ 
candescence ; in other words, to change its vibrating period in the direction of 
a ur/m ented ref rang ib ility . * 

The memoir concludes with some remarks on the connexion of radiation and 
conduction, and experiments are adduced which indicate that they are reciprocal 
phenomena. 

* The late sagacious Dr. William Allen Miller drew this inference, and published it in 
1855. Until informed of it by himself after the publication of the present memoir, I was 
ignorant of his having done so. The conclusion here arrived at with regard to radiant heat 
is abundantly proved in Memoir VIII. 




VI. 

CONTRIBUTIONS TO MOLECULAR PHYSICS. 


The Bakerian Lecture delivered before the Royal Society , 

March 17, 1864 * 


§ 1 . 

Preliminary Considerations.—Description of Apparatus. 

The natural philosophy of the future must, I imagine, mainly 
consist in the investigation of the relations which subsist 
between the ordinary matter of the universe and the sether in 
which this matter is immersed. Regarding the motions of the 
sether itself, as illustrated by the phenomena of reflexion, re¬ 
fraction, interference, and diffraction, the optical investigations 
of the last half-century have left nothing to be desired ; but as 
regards the atoms and molecules which take up, and from which 
issue, the undulations of light and heat, and the relations of 
those atoms and molecules to the medium which they move, 
and by which they are set in motion, these investigations teach 
us little. To come closer to the origin of the sethereal waves 
—to get, if possible, some experimental hold of the oscillating 
atoms themselves—has been the main object of the researches 
in which I have been engaged for the last five years. In these 
researches radiant heat has been used as an instrument for 
exploring molecular condition, and this also is the object kept 
constantly in view throughout the investigation, the results of 
which I have now the honour to submit to the scientific public. 

* Philosophical Transactions for 1864, p. 327. Philosophical Magazine , December 
1864. 




200 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


The first part of these researches is devoted to the more com¬ 
plete examination of a subject which was briefly touched upon 
at the conclusion of the last memoir—namely, the action of 
liquids, as compared with that of their vapours, upon radiant 
heat. The differences which exist between different gaseous 
molecules, as regards their power of emitting and absorbing 
radiant heat, have been already amply illustrated. When a gas 
is condensed to a liquid, the molecules approach and grapple 
with each other by forces which are insensible as long as the 
gaseous state is maintained. But though thus condensed and 
enthralled, the sether still surrounds the molecules. If, then, 
the powers of radiation and absorption depend mainly upon 
them individually, we may expect that the deportment towards 
radiant heat which experiment establishes in the case of mole¬ 
cules in a state of gaseous freedom, will maintain itself after 
the molecule has relinquished its freedom and formed part of 
a liquid. If, on the other hand, the state of aggregation be 
of paramount importance, we may expect to find on the part of 
liquids a deportment altogether different from that of the 
vapours from which they are derived. 

Melloni, it is well known, examined the diathermancy of 
various liquids, but he employed for this purpose the flame of 
an oil-lamp, covered by a glass chimney. His liquids, moreover, 
w T ere contained in glass cells ; hence the radiation from the 
source was profoundly modified before it entered the liquid at 
all, for the glass was impervious to a considerable part of the 
radiation. It was not only my wish to interfere as little as 
possible with the primitive emission, but it was also my aim 
to compare the action of liquids with that of their vapours, 
examined in a tube stopped with plates of rock-salt. I 
therefore devised an apparatus in which layers of liquid of 
variable thickness could be enclosed between two polished 
plates of rock-salt. It was skilfully constructed for me by Mr. 
Becker, and the same two plates have already done service in 
more than six hundred experiments. 

The ajvparatus consists of the following parts :—ABC (fig. 16) 
is a plate of brass, 3*4 inches long, 2-1 inches wide, and 0*3 of 
an inch thick. Into it, at its corners, are rigidly fixed four 
upright pillars, furnished at the top with threads, for the recep¬ 
tion of the nuts q r s t. D E F is a second plate of brass of the 



CONTRIBUTIONS TO MOLECULAR PHYSICS 


201 


same size as the former, and pierced with holes at its four cor¬ 
ners, so as to enable it to slip over the four columns of the plate 
ABC. Both these plates are perforated by circular apertures, 


r-H 

W) 

K 






m n and op, 1-35 inch in diameter. G II I is a third plate of 
brass of the same area as D E F, and, like it, having its centre 





































































202 CONTRIBUTIONS TO MOLECULAR PHYSICS. 

and its corners perforated. It is intended to come between 
the two plates of rock-salt, which are to form the walls of 
the cell, and its thickness determines that of the liquid layer. 
Thus when the plates ABC and D E F are in position, a space 
of the form of a shallow cylinder is enclosed between them, and 
this space can be filled with any liquid through the orifice Jc. 
The separating plate GHI was ground with the utmost accu¬ 
racy, and the surfaces of the plates of salt were polishe'd with 
extreme care, with a view to rendering the contact between the 
salt and the brass water-tight. In practice, however, it was 
found necessary to introduce washers of thin letter-paper 
between the plates of salt and the separating plate. 

In arranging the cell for experiment, the nuts qr s t are 
unscrewed, and a washer of india-rubber is first placed on 
ABC. On this washer is placed one of the plates of rock- 
salt. On the salt is placed the washer of letter-paper, and on 
this again the separating plate GHI. A second washer of 
paper is placed on this plate; then comes the second plate of 
salt, on which another india-rubber washer is laid. The plate 
D E F is finally slipped over the columns, and the whole 
arrangement is tightly screwed together by the nuts q r s t. 
The use of the india-rubber washers is to relieve the crushing 
pressure which would be applied to the plates of -salt if they 
were in actual contact with the brass plates ; and the use of the 
paper washers is, as already explained, to render the cell liquid- 
tight. After each experiment, the apparatus is unscrewed, the 
plates of salt are removed and thoroughly cleansed; the cell is 
then remounted, and in two or three minutes all is ready for a 
new experiment. 

My next necessity was a perfectly steady source of heat, of 
sufficient intensity to penetrate the most powerfully absorbent 
liquid. This was found in a spiral of platinum wire, rendered 
incandescent by an electric current. The frequent use of this 
source of heat led me to construct the lamp shown in fig. 17. 
A is a globe of glass 3 inches in diameter, fixed upon a stand, 
which can be raised and lowered. At the top of the globe is a 
tubulure, into which a cork is fitted, and through the cork pass 
two wires whose ends are united by the platinum spiral s. The 
wires are earned down to the binding-screws a b, which are 
fixed in the foot of the stand, so that when the instrument is 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


203 


attached to the battery no strain is ever exerted on the wires 
which carry the spiral. The ends of the thick wire to which the 
spiral is attached are also of stout platinum; for when it was 
attached to copper wires, unsteadiness was ultimately intro¬ 
duced through oxidation. The heat from the incandescent 
spiral issues by the opening d , which is an inch and a half in 
diameter. Behind the spiral, finally, is a metallic reflector, r, 
which augments the flux of heat without sensibly changing its 
quality. In the open air the red-hot spiral is a capricious 
source of heat; but surrounded by its glass globe its steadiness 
is admirable. 

The whole experimental arrangement will be immediately 
understood from the sketch given in fig. 18. A is the platinum 
lamp just described, heated by a current from a Grove’s 
battery of five cells. It is necessary that this lamp should 
remain perfectly constant throughout the day; and to keep it 
so, a tangent galvanometer and a rheocord are introduced into 
the circuit. 

In front of the spiral, and surrounding the tubulure of its 
globe, is the tube B with an interior reflecting surface, through 
which the heat passes to the rock-salt cell C. This cell is 
placed on a little stage soldered to the back of the perforated 
screen S S', so that the heat, after having crossed the cell, passes 
through the hole in the screen, and afterwards impinges on the 
thermo-electric pile P. The pile is placed at some distance 
from the screen S S', so as to render the temperature of the 
cell C itself of no account. C' is the compensating cube, con¬ 
taining water kept boiling by steam from the pipe p. Between 
the cube C' and the pile P is the screen Q, which regulates the 
amount of heat falling on the posterior face of the pile. The 
whole arrangement is here exposed, but in practice the pile P 
and the cube C' are carefully protected from the capricious 
action of the surrounding air. 

The experiments are thus performed. The empty rock-salt 
cell C being placed on its stage, a double silvered screen (not 
shown in the figure) is first introduced between the end of the 
tube B and the cell C—the radiation from the spiral being thus 
totally cut off, and the pile subjected to the action of the cube 
C' alone. By means of the screen Q, the total heat to be 
adopted throughout the series of experiments is obtained: say 


204 CONTRIBUTIONS TO MOLECULAR PHYSICS. 

that it is sufficient to produce a galvanometric deflection of 50 
degrees. The double screen used to intercept the radiation from 



the spiral is then gradually withdrawn until this radiation com¬ 
pletely neutralizes that from the cube, and the needle of the 









































































CONTRIBUTIONS TO MOLECULAR PHYSICS. 


205 


galvanometer points steadily to zero. The position of the double 
screen, once fixed, remains subsequently unchanged, the slight 
and slow alteration of the source of heat being neutralized by the 
rheocord. Thus the rays in the first instance pass from the spiral 
through the empty rock-salt cell. A small funnel, supported 
by a suitable stand, dips into the aperture leading into the 
cell, and through this the liquid is poured. The introduction 
of the liquid destroys the previous equilibrium, the galvano¬ 
meter needle moves, and finally assumes a steady deflection; 
and from this deflection we can immediately calculate the quan¬ 
tity of heat absorbed by the liquid, and express it in hundredths 
of the entire radiation. 

For example, the empty cell being placed upon its stand, and 
the needle being at 0°, the introduction of iodide of methyl 
produced a deflection of 30*8°. The total radiation on this 
occasion was 44*2°. Taking the force necessary to move the 
needle from 0° to 1° as our unit, the deflection 30*8° corre¬ 
sponds to 32 such units, while the deflection 44*2° corresponds 
to 58-3 such units. Hence the statement 

58-3 : 100 = 32 : 54 - 9 , 

which gives an absorption of 54*9 per cent, for a-flayer of liquid 
iodide of methyl 0*07 of an inch in thickness. 


§ 2 . 


Absorption of Radiant Heat of a certain Quality by eleven different 
Liquids*at five different Thicknesses. 

The following table contains the results obtained in this 
manner with the respective liquids there mentioned. It em¬ 
braces both the deflection produced by the introduction of the 
liquid, and the quantity per cent, intercepted of the entire 
radiation. 

It has been intimated to me that the publication of such 
details as would enable a reader to judge of the precision 
attainable by my apparatus would be desirable. In this paper, 
I, to some extent, endeavour to meet this desire, without, how¬ 
ever, quitting the ordinary course of experiment. 


206 


CONTRIBUTION'S TO MOLECULAR PHYSICS. 


Table I .—Radiation of Heat through Liquids. 

Source of heat: red-hot 'platinum spiral. 

Thickness of liquid layer, 0 - 07 of an inch. 


Name of Liquid 



Deflection 

Absorption per 100 

Iodide of methyl . 



33'5 

53-7 

Iodide of ethyl 



35-5 

587 

Benzol 



375 

64-4 

Amylene 



39o 

70 7 

Sulphuric ether 



41 

75-4 

Acetic ether . 



415 

769 

Formic ether 



42-4 

80 

Alcohol 



43-5 

84-2 

Water . 



447 

90-5 

Total heat . 



467 

100 


In these experiments a far less delicate galvanometer was 
employed than that used in my former researches. The experi¬ 
ments were made on September 29, 1863, and on the following 
day they were repeated with the same result. 

On October 28 my most delicate galvanometer was at liberty, 
and with it I executed the experiments performed with the 
coarser one. The following are the results :— 


Table II .—Radiation of Heat through Liquids. 


Source of heat: red-hot platinum spiral. 
Thickness of liquid layer, 0'07 of.an inch. 


Name of Liquid 

Bisulphide of carbon . 

Chloroform ..... 

Iodide of methyl .... 

Ditto, strongly coloured with iodine 
Iodide of ethyl .... 

Benzol...... 

Amylene ..... 

Sulphuric ether .... 

Acetic ether 

Formic ether .... 

Alcohol ..... 

Water ...... 

Total heat. 


Deflection 

Absorption 
per 100 

0 

9 

12-5 


25-2 

35 


36 

53-2 

53-7 

36 

532 


38-2 

59 

58-7 

39-2 

62-5 

644 

42 

736 

70-7 

42-6 

76-1 

75-4 

434 

78 

76-9 

433 

79 

80 

44-4 

83-6 

842 

45-6 

8S-8 

90-5 

48 

100 











CONTRIBUTIONS TO MOLECULAR PHYSICS. 


207 


Beside the results obtained with the delicate galvanometer 
are placed those obtained with the coarser one. It is not my 
object to push these measurements to the last degree of nicety; 
otherwise the satisfactory agreement here exhibited might be 
made still more exact. 

The following series of tables contain the results obtained 
with liquid layers of various thicknesses, employing throughout 
my most delicate galvanometer :— 


Table III. — Radiation of Heat through Liquids. 

Source of heat: red-hot ‘platinum spiral. 

Thickness of liquid layer, 0'02 of an inch. 


Name of Liquid 


Deflection 

Absorption per 100 

Bisulphide of carbon 


o 

4 

5’5 

Chloroform . 


12 

16-6 

Iodide of methyl . 


26 

36-1 

Iodide of ethyl 


27-5 

• 

38-2 

Benzol.... 


31-3 

43-4 

Amylene 


38 

58-3 

Boracic ether 


39 

61-8 

Sulphuric ether 


39-5 

63-3 

Formic ether 


40 

6o-2 

Alcohol 


40-5 

67-3 

Water .... 


43*7 

807 

Total heat . 


48 

100 


Table IY. — Radiation of Heat through Liquids. 

Source of heat: red-hot platinum spiral. 


Thickness of liquid layer, 

0 - 04 of an inch. 


Name of Liquid 

Deflection Absorption per 100 

Bisulphide of carbon 

o 

6-1 

8-4 

Chloroform. 

18 

25 

Iodide of methyl .... 

33 

46*5 

Iodide of ethyl .... 

35 

507 

Benzol ...... 

37 

557 

Amylene ..... 

40 

65*2 

Boracic ether .... 

41 

69-4 

Sulphuric ether .... 

42 

73’5 

Acetic ether ..... 

42-1 

74 

Formic ether .... 

42*5 

76-3 

Alcohol ..... 

43-2 

78-6 

Water ...... 

45 

86*1 

Total heat. 

48 

100 










208 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Table Y. — Radiation of Heat through Liquids. 


Source of heat: red-hot 'platinum spiral. 
Thickness of liquid layer, 0'14 of an inch. 


Name of Liquid 

Deflection 

Absorption per 100 

Bisulphide of carbon 

o 

11 

15-2 

Chloroform .... 

28-6 

40 

Iodide of methyl . 

40 

65-2 

Iodide of ethyl . . 

40-9 

69 

Benzol ..... 

41-5 

71-5 

Amylene .... 

43 

777 

Sulphuric ether 

43-2 

78-6 

Acetic ether .... 

44 

82- 

Formic ether 

44-5 

84 

Alcohol .... 

44-8 

85-3 

Water ..... 

463 

91-8 

Total heat .... 

48 

100 


Table VI. — Radiation of Heat through Liquids. 


Source of heat: red-hot platinum spiral. 
Thickness of liquid layer, 0'27 of an inch. 


Name of Liquid 


Deflection 

Absorption per 100 

Bisulphide of carbon 


o 

12-5 

17-3 

Chloroform . 


323 

44-8 

Iodide of methyl . 


40-8 

68-6 

Iodide of ethyl 


41-5 

71*5 

Benzol .... 


42 

73*6 

Amylene 


44-1 * 

82*3 

Sulphuric ether 


44-8 

85-2 

Acetic ether . 


45 

86T 

Formic ether 


. 45-2 

87 

Alcohol 


45-7 

89Y 

Water .... 


46-1 

91 

Total heat . 


48 

100 


The foregoing results are collected together in the following 
table:— 










CONTRIBUTION'S TO MOLECULAR PHYSICS. 


209 


Table YII.— Absorption of Heat by Liquids. 

Source of heat: Platinum Spiral heated to a bright redness by a voltaic current. 


Liquid 

Thickness of liquid in parts of an inch 

0-02 

0-04 

0*07 

0-14 

0-27 

Bisulphide of carbon 


5-5 

8-4 

12*5 

15-2 

17-3 

Chloroform 


16-6 

25-0 

35-0 

40-0 

44-8 

Iodide of methyl 


36 1 

46-5 

53-2 

65-2 

68-6 

Iodide of ethyl 


38-2 

50-7 

59‘0 

69-0 

71-5 

Benzol .... 


43-4 

557 

62-5 

71-5 

73 6 

Amylene 


58-3 

65-2 

73-6 

777 

82-3 

Sulphuric ether 


633 

73-5 

76-1 

78-6 

85-2 

Acetic ether . 



74-0 

78-0 

82-0 

86-1 

Formic ether . 


65-2 

76-3 

79-0 

840 

87-0 

Alcohol .... 


67-3 

78-6 

83-6 

85-3 

89-1 

Water .... 


807 

861 

888 

91-8 

91-0 


Had it been desirable to push these measurements to the 
utmost limit of accuracy, I should have repeated each experi¬ 
ment, and taken the mean of the determinations. But consi¬ 
dering the way in which the different thicknesses check each 
other, an inspection of the table must produce the conviction 
that the results express, within small limits of error, the action 
of the bodies mentioned. The order of absorption is certainly 
that here shown. 


§ 3. 

Absorption of Radiant Heat of the same quality by the Vapours 
of these Liquids at a common Pressure. 


As liquids, then, those bodies are shown to possess very 
different capacities ob intercepting the heat emitted by our 
radiating Source; and we have next to inquire whether these 
differences continue after the molecules have been released from 
the bond of cohesion. We must, of course, test vapours and 
liquids by waves of the same period; and this our mode of 
experiment renders easy of accomplishment. The heat gene¬ 
rated in a wire by a current of a given strength being invari¬ 
able, it was only necessary, by means of the tangent compass 
and rlieocord, to keep the current constant from day to day 
in order to obtain, both as regards quantity and quality, an 
invariable source of heat. 

The liquids from which the vapours were derived were placed 
in small long flasks, a separate flask being devoted to each. The 
14 




















210 CONTRIBUTIONS TO* MOLECULAR PHYSICS. 

air above the liquid and within it being first carefully removed 
by an air-pump, the flask was attached to the experimental tube. 
This was of brass, 49*6 inches long, and 2*4 inches in diameter, 
its two ends being stopped by plates of rock-salt. Its interior 
surface was polished. At the commencement of each experi¬ 
ment, the tube having been thoroughly cleansed and exhausted, 
the needle stood at zero.* The cock of the flask containing 
the volatile liquid was then carefully turned on, and the vapour 
allowed slowly to enter the experimental tube. The barometer 
attached to the tube was finely graduated, and the descent of 
the mercurial column was observed through a magnifying lens. 
When a pressure of 0*5 of an inch was obtained, the vapour 
was cut off and the permanent deflection of the needle noted. 
Knowing the total heat, the absorption in lOOths of the entire 
radiation could be at once deduced from the deflection. The 
following table contains the results of a series of experiments 

made with the platinum spiral as source :— 

\ 

Table YIII.— Radiation of Heat through Vapours. 

Source of heat: red-hot Platinum Spiral. 

Pressure, 0'5 of an inch. 


Name of Vapour 

Deflection 

Absorption per 100 

Bisulphide of carbon 

o 

16-5 

4-7 

Chloroform .... 

22-8 

6*5 

Iodide of methyl . 

33 

96 

Iodide of ethyl 

45 

17-7 

Benzol . . • . 

48 

20-6 

Amylene .... 

553 

27-5 

Alcohol .... 

557 

2^1 

Formic ether 

58-2 

31*4 

Sulphuric ether 

58-5 

31-9 

Acetic ether .... 

59-9 

34-6 

Total heat .... 

78 

100 


§ 4. 

Order of Absorption of Liquids at a common Thickness , and 
Vapours at a common Pressure. 

We are now in a condition to compare the action of a series 
of volatile liquids at a common thickness with that of the 

* It is hardly necessary to remark that the principle of compensation described in 
my former memoirs was employed here also. 






CONTRIBUTIONS TO MOLECULAR PHYSICS. 211 


vapours of tliose liquids at a common pressure upon radiant 
heat. 

Commencing with the substance of the lowest absorptive 
energy, and proceeding to the highest, we have the following 
order of absorption :— • 


Liquids 

Bisulphide of carbon. 
Chloroform. 

Iodide of methyl. 
Iodide of ethyl. 
Benzol. 

Amylene. 

Sulphuric ether. 
Acetic ether. 

Formic ether. 

Alcohol. 

Water. 


Vapours 

Bisulphide of carbon. 
Chloroform. 

Iodide of methyl. 
Iodide of ethyl. 
Benzol. 

Amylene. 

Alcohol. 

Formic ether. 
Sulphuric ether. 
Acetic ether. 


Here, as far as amylene, the order of absorption is the same 
for both liquids and vapours. But from amylene downwards, 
though strong liquid absorption is in a general way paralleled 
by strong vapour absorption, the order of both is not the same. 
There is not the slightest doubt that next to water alcohol is 
the most powerful absorber in the list of liquids ; but there is 
just as little doubt that the position which it occupies in the 
list of vapours is the correct one. This has been established 
by reiterated experiments. Acetic ether, on the other hand, 
though certainly the most energetic absorber in the state of 
vapour, falls behind both formic ether and alcohol in the liquid 
state. Still, on the whole, I think it is inrpossible to contem¬ 
plate these results without arriving at the conclusion, that 
the act of absorption is in the main molecular, and that the 
molecule maintains its power as an absorber and radiator when 
it changes its state of aggregation. Should, however, any 
doubt linger as to the correctness of this conclusion, it will 
speedily disappear. 



Order of Absorption of Liquids and Vapours in proportional 

Quantities. 

A moment’s reflexion will show that the comparison insti¬ 
tuted in the last section is not a strict one. We have taken the 


212 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


liquids at a common thickness, and the vapours at a common 
volume and pressure. But if the layers of liquid employed were 
turned bodily into vapour, the volumes obtained would not he 
the same. Hence the quantities of matter traversed by the 
radiant heat are neither equal nor'proportional to each other in 
the two cases; and to render the comparison strict they ought 
to be proportional. It is easy, of course, to make them so ; for 
the liquids being examined at a constant volume, their specific 
gravities give us the relative quantities of matter traversed by 
the radiant heat, and from these and the vapour-densities we 
can immediately deduce the corresponding volumes of the 
vapour. Calling the quantity of matter q, the vapour-density d, 
and the volume Y, we have 


Yd = q, 
or 

Dividing, therefore, the specific gravities of our liquids by the 
densities of their vapours, we obtain a series of volumes propor¬ 
tional to the masses of liquid employed. The densities of both 
liquids and vapours are given in the following table:— 


IX .—Table of Densities. 



Vapour 

Liquid 

Bisulphide of carbon 

2-63 

1-27 

Chloroform .... 

4-13 

1-48 

Iodide of methyl . 

4-90 

224 

Iodide of ethyl 

5-39 

1-95 

Benzol ..... 

2-69 

0-85 

Amylene .... 

242 

0-64 

Alcohol .... 

1-59 

0-79 

Sulphuric ether 

2-56 

0-71 

Formic ether 

2-56 

0-91 

Acetic ether .... 

3-04 

089 

Water ..... 

0-63 

1 

Air. 

1 



Substituting for q the numbers of the second column, and for 
d those of the first, we obtain the following series of vapour 
volumes, whose weights are proportional to the masses of liquid 
employed :— 






CONTRIBUTIONS TO MOLECULAR PHYSICS. 


213 


X. — Table of Proportional 

Volumes. 

Bisulphide of carbon . 

0-48 

Chloroform. . . * . 

0-36 

Iodide of methyl 

0-46 

Iodide of ethyl .... 

0-36 

Benzol ..... 

0-32 

Amylene ..... 

0*26 

Alcohol ..... 

0-50 

Sulphuric ether .... 

0-28 

Formic ether .... 

0-36 

Acetic ether .... 

. 0-29 

Water 

1-60 


Employing the vapours in the volumes here indicated, the 
following results were obtained :— 


Table XI.— Radiation of Heat through Vapours. 

Mass of Vapour proportional to Mass of Liquid. 


Name of Vapour 

Pressure in parts 
of an inch 

Deflection 

Absorption 
per 100 

Bisulphide of carbon . 

0-48 

r 8-4i 
l 8-5 J 

4-3 

Chloroform. 

0-36 

f 13 d 
\ 13 / 

6-6 

Iodide of methyl 

. . 0-46 

f2° d 
\20-4J 

10-2 

Iodide of ethyl . 

0*36 

/ 30*61 
\30-6J 

15 

Benzol 

0-32 

/ 33-41 

U 33*1 / 

16-8 

Amylene . 

. . 0-26 

37-7 

19 

Sulphuric ether . 

. . 0-28 

/ 42-51 

1 42*6 / 

21*5 * 

Acetic ether 

. . 0-29 

/ 44 d 

144 / 

22*2 

Formic ether 

0-36 

5 44-51 
l 44*7 / 

22-5 

Alcohol 

0-50 

(45 } 

l 44-9 > 

22*7 


Here the discrepancies revealed by our former series of ex¬ 
periments entirely disappear, and it is proved that for heat of 
the same quality the order of absorption for liquids and their 
vapours is the same. We may therefore safely infer that the 
position of a vapour as an absorber or radiator is determined 
by that of the liquid from which it is derived. Granting the 
validity of this inference, the position of water fixes that of 
aqueous vapour. From the first seven tables of this memoir, or 
from the resume of results in Table VII., it will be seen that 










214 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


for all thicknesses water exceeds the other liquids in the energy 
of its absorption. Hence, if no single experiment on the vapour 
of water existed, we should be compelled to conclude, from the 
deportment of its liquid, that, weight for weight, aqueous 
vapour transcends all others in absorptive power. Add to this 
the direct and multiplied experiments by which the action of 
this substance on radiant heat has been established, and we 
have before us a body of evidence sufficient, I trust, to set this 
question for ever at rest, and to induce the meteorologist to 
apply without misgiving the radiant and absorbent property of 
aqueous vapour to the phenomena of his science. 


§ 6 . 

Remarks on the Chemical Constitution of Bodies with reference to 

their Powers of Absorption. 

The order and relative powers of absorption of our vapours, 
when equal volumes are compared, are given in Table VIII. : the 
chemical formulae of the substances, and the number of atoms 
which the molecules embrace, are as follows :— 



Formula NU j™Molecules 1113 

Bisulphide of Carbon . 

. C S 2 

3 

Chloroform . 

. ‘ . CHC1 3 

5 

Iodide of methyl 

. CffI 

5 

Iodide of ethyl 

. . C 2 H 5 1 

8 

Benzol 

. C 6 H 6 

12 

Amylene 

. C 5 H 10 

15 

Alcohol 

. . . C 2 H 6 0 

9 

Formic ether 

. C 3 H 6 O 2 

11 

Sulphuric ether . 

. C* H 10 0 

15 

Acetic ether. 

. C* H 8 O 2 

14 

Boracic ether 

. B C 6 H’ s O 3 

25 


Here for the first six vapours, the radiant and absorbent 
powers augment with the number of atoms contained in the 
molecules. Alcohol and amylene vapours, however, are nearly 
alike in absorptive power, the molecule of amylene containing 
15 atoms while that of alcohol embraces only 9. But in alcohol 
we have a third element introduced, which is absent in the amy¬ 
lene ; the oxygen of the alcohol gives its molecule a character 
which enables it to transcend the amylene molecule, though the 
latter contains the greater number of atoms. Here the idea of 









CONTRIBUTIONS TO MOLECULAR PHYSICS. 


215 


quality superadds itself to that of number. Acetic ether also 
has a smaller number of atoms in its molecule than sulphuric 
ether; the latter, however, has but one atom of oxygen, while 
the former has two. Formic ether and sulphuric ether are 
almost identical in their absorptive powers for the heat here 
employed ; still formic ether has but 11 atoms in its molecule, 
-while sulphuric has 15. But formic ether possesses two atoms 
of oxygen, while sulphuric possesses only one. Two things 
seem influential on the absorbent and radiant power, which 
may be expressed by the terms multitude and complexity. As a 
molecule of multitude, amylene, for example, exceeds alcohol; 
as a molecule of complexity, alcohol exceeds amylene; and in 
this case, as regards radiant and absorbent power, the com¬ 
plexity is more than a match for the multitude. The same 
remarks apply to sulphuric and formic ether: the former excels 
in multitude, the latter in complexity, the excess in the one 
case almost exactly balancing that in the other. Adding two 
atoms of hydrogen and one of carbon to formic ether, we 
obtain acetic ether, and by this addition the balance is turned; 
for though acetic ether falls short of sulphuric ether in multi¬ 
tude, it transcends it in absorbent and radiant power. Out¬ 
standing from all others, when equal volumes are compared, 
and signalizing itself by the magnitude of its absorption, we 
have boracic ether, each molecule of which embraces no less 
than 25 atoms. The time now at my disposal enables me to do 
little more than glance at these singular facts ; but, in passing, 
I must direct the attention of chemists to the water-molecule 
its power as a radiant and an absorbent is perfectly unpre¬ 
cedented and anomalous, if the usually recognized formula be 
correct. 


§ 7 . 


Transmission of Radiant Heat through Bodies opaque to Light .— 
Remarks on the Physical Cause of Transparency and Opacity. 


In Table II. a fact is revealed which is worth a little 
further attention. The measurements there recorded show that 
the absorption of a layer of iodide of methyl, strongly coloured 
with iodine (which had been liberated by the action of light) 
was precisely the same as that of a perfectly transparent layer 


216 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


of the liquid. The iodine, which produced so marked an effect 
on light, did not sensibly affect the radiant heat emitted by the 
platinum spiral. Here are the numbers :— 

Absorption per 100 

Iodide of methyl (transparent) . . . . . .53*2 
Iodide of methyl (strongly coloured with iodine) . . 53*2 

In this case, the incandescent platinum spiral, or a bright 
flame, was visible when looked at through the liquid ; I there¬ 
fore intentionally deepened the colour (a rich brown), by adding 
iodine, until the layer was able to cut off wholly the light of 
a brilliant jet of gas. The transparency of the liquid to the 
radiant heat was not sensibly affected by the addition of the 
iodine. The luminous heat was of course cut off; but this, as 
compared with the whole radiation, was so small as to be insen¬ 
sible in the experiments. 

It is known that iodine dissolves freely in the bisulphide of 
carbon, the colour of the solution in thin layers being a splendid 
purple; but in layers of moderate thickness it may be rendered 
perfectly opaque to light. I dissolved in the liquid a quantity of 
the iodine sufficient, when introduced into a cell 0*07 of an inch 
in width, to cut off wholly the light of the most brilliant gas- 
flame. Comparing the opaque solution with the transparent 
bisulphide, the following results were obtained :— 

Deflection Absorption per 100 

Bisulphide of carbon (opaque) ... 9° 12*5 

Bisulphide of carbon (transparent). . . 9° 12*5 

• • 

Here the presence of a quantity of iodine, perfectly opaque to a 
brilliant light, was without measurable effect upon the heat 
emanating from our platinum spiral. The liquid was sensibly 
thickened by the quantity of iodine dissolved in it. 

The same liquid was placed in a cell 0*27 of an inch in width; 
that is to say, a solution which was perfectly opaque to light at 
a thickness of 0-07, was employed in a layer of nearly four times 
this thickness. Here are the results :— 

Deflection Absorption per 100 

Bisulphide of carbon (transparent). . . 13*6° 18*8 

Bisulphide of carbon (opaque) . . .13*7° 19 

The difference between the two measurements lies within the 
limits of possible error. 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


217 


Bisulphide of carbon is commonly used to fill hollow prisms, 
when considerable dispersion is desired in the decomposition of 
white light. Such prisms, filled with the opaque solution, inter¬ 
cept entirely the luminous part of the spectrum, but allow the 
ultra-red rays free passage. A lieat-spectrum of the sun, or of 
the electric light may be thus obtained entirely separated from 
the luminous one. By means of a prism of the transparent 
bisulphide, I determined the position of the spectrum of the 
electric light upon a screen, and behind the screen placed a 
thermo-electric pile, so that when the screen was removed the 
ultra-red rays fell upon the pile. I then substituted an opaque 
prism for the transparent one : there was no visible spectrum 
on the screen; but its removal at once demonstrated the 
existence of an invisible spectrum by the thermo-electric 
current which it generated, and which was powerful enough 
to dash violently aside the needles of a large lecture-room 
galvanometer. 

To what, then, are we to ascribe the deportment of iodine 
towards luminous and obscure heat ? The difference between 
both qualities of heat is simply one of period. In the one case 
the waves which convey the energy are short and of rapid re¬ 
currence ; in the other case they are long and of slow recur¬ 
rence ; the former are intercepted by the iodine, and the latter 
are allowed to pass. Why ? There can, I think, be only one 
answer to this question—that the intercepted waves are those 
whose periods coincide with those of the atoms of the dissolved 
iodine. Supposing waves of any period to impinge upon an 
assemblage of molecules of any other period, it is, I think, 
physically certain that a tremor of greater or less intensity will 
be set up among the molecules; but for the motion to accu¬ 
mulate so as to produce sensible absorption, coincidence of 
period is necessary. Briefly defined, therefore, transparency is 
synonymous with discord , while opacity is synonymous with 
accord between the periods of the waves of aether and those of 
the molecules of the body on which they impinge. The trans¬ 
parency, then, of our solution of iodine to the ultra-red undu¬ 
lations demonstrates the incompetency of its atoms to vibrate 
in unison with the longer waves. 

This simple conception will, I think, be found sufficient to 
conduct us with intellectual clearness through a multitude of 


218 CONTRIBUTIONS TO MOLECULAR PHYSICS. 

otherwise perplexing phenomena. It may of course be applied 
immediately to that numerous class of bodies which are transpa¬ 
rent to light, but opaque in a greater or less degree to radiant 
heat. Water, for example, is an eminent example of this class 
of bodies : while it allows the luminous rays to pass with free¬ 
dom, it is highly opaque to all radiations emanating from obscure 
sources of heat. A layer of this substance one-twentieth of an 
inch thick is competent, as Mellon! has shown, to intercept all 
rays issuing from bodies heated under incandescence. Hence 
we may infer that, throughout the range of the visible spectrum, 
the periods of the water-molecules are in discord with those of 
the sethereal waves, while beyond the red we have coincidence 
between both. 

What is true of water is, of course, true in a less degree of 
glass, alum, calcareous spar, and of the various liquids named 
in the first section of this paper. They are all in discord with 
the visible spectrum ; they are all more or less in accord with 
the ultra-red undulations of the spectrum. 

Thus also as regards lampblack: the blackness of the sub¬ 
stance is due to the accord which reigns between the oscillating 
periods of its atoms and those of the waves embraced within 
the limits of the visible spectrum. The substance which is 
thus impervious to the luminous rays is moreover the very one 
from which the whitest light of our lamps is derived. . It can 
absorb all the rays of the visible spectrum; it can also emit 
them. But though in a far less degree than iodine, lampblack 
is also to some extent transparent to the longer undulations. 
Mellon! was the first to prove this ; and an experiment de¬ 
scribed in a former memoir proved that 30 per cent, of the 
radiation from an obscure source of heat found its way through 
a layer of lampblack which cut off* totally the light of the most 
brilliant jet of gas. I shall have occasion to show that, for 
certain sources of heat of long period, between 40 and 50 per 
cent, of the entire radiation is transmitted by a layer of lamp¬ 
black which is perfectly opaque to our most brilliant artificial 
lights. Hence, in the case of lampblack, while accord exists 
between the periods of its atoms and those of the light¬ 
exciting waves, discord, to a considerable extent, exists be¬ 
tween the periods of the same atoms and those of the ultra- 
red undulations. 






CONTRIBUTIONS TO MOLECULAR PHYSICS. 


219 


§ 8 . 

Influence of the Temperature of the Source of Heat on the Trans¬ 
mission of Radiant Heat. 

To obtain sources of heat of different temperatures, Melloni 
resorted to lamps, to spirals heated to incandescence by the 
flame of alcohol, to copper laminae heated by flame, and to the 
surfaces of vessels containing boiling water. No conclusions 
regarding temperature can, as will afterwards be shown, be 
drawn from such experiments; but by means of the platinum 
spiral we can go through all those changes of temperature, 
retaining throughout the same vibrating atoms , and we can there¬ 
fore investigate how the alteration of the rate of vibration 
affects the rate of absorption. The following series of ex¬ 
periments were executed on the 9th of October, with a plati¬ 
num spiral raised to barely visible redness, and vapours at 
a pressure of 0*5 of an inch :— 


Table XII .—Radiation of Heat through Vapours. 


Source of Heat: Platinum Spiral barely visible in the dark. 


Name of Vapour 




Deflection 

Absorption per 100 

Bisulphide of carhon 




o 

. 7-5 

6*51 

Bisulphide of carbon 




. 7*45 

6-4 J 

Chloroform 




. 10-5 

®n 

Chloroform 




. 10-5 

9-1 J 

Iodide of methyl 




. 145 

12-51 

Iodide of methyl 




. 14-5 

12-5 J 

Iodide of ethyl . 




. 24*2 

20-91 

> 

Iodide of ethyl . 




. 24’5 

21-1 J 

Benzol 




. 31-0 

26-71 

Benzol 




. 30 

25-9 1 

Amylene . 




. 37-6 

35-6 1 

Amylene . 




. 37-8 

3o-9 J 

Sulphuric ether . 




. 41-1 

43*4 \ 

Sulphuric ether . 




. 41 

43-4 J 

Formic ether . . 




. 41-7 

45 i 

Formic ether . . 




. 41-8 

45-3J 

Acetic ether . . 




. 43-6 

49-81 

Acetic ether . . 

0 

• 

* 

, 43-4 

49-3 J 












220 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


On the 10th of October the following results were obtained 
with the same platinum spiral, raised to a white heat:— 


Table XIII .—Radiation of Heat through Vapours. 

Source of heat: White-hot Platinum Spiral. 


Name of Vapour 

Bisulphide of carbon 




Deflection 

o 

. 3-5 

Absorption per 100 

2-9 \ 

Bisulphide of carbon 




. 3-4 

2-8 J 

Chloroform 




. 67 

. 

Chloroform 




. 6-7 

5-6 J 

Iodide of methyl 




. 9-2 

77 \ 

Iodide of methyl 




. 9*4 

7-9 J 

Iodide of ethyl. 




. 15-4 

13 \ 

Iodide of ethyl. 




. 15 

12-6/ 

Benzol 




. 19-3 

16*6 \ 

Benzol 




. 19 

16-41 

Total heat. 




. 59-2 

100 

Amylene . . . 




. 27*6 

22-6 y 

Amylene . 



* • 

. 27 7 

22-7 / 

Formic ether 




. 305 

25 y . 

Formic ether 




. 30-7 

25-2/ 

Sulpthuric ether . 




. 31-4 

25*7 ^ 

Sulphuric ether . 




. 317 

26-0/ 

Acetic ether 




. 33 

27 \ 

27 3 J 

Acetic ether 




. 332 

Total heat. 




. 60 

4 

100 


With the same spiral, brought still nearer to its point of 
fusion, and with four of the vapours, the following results were 
obtained:— 

Table XIY. — Radiation of Heat through Vapours. 

Source of heat: Platinum Spiral at an intense white heat. 


Name of Vapour 



Deflection 

Absorption per 100 

Bisulphide of carbon 



o 

. 14-5 

2 , 5'l 

Bisulphide of carbon 



. 14-5 

25/ 

Chloroform 

• 



. 23 

3 9 T 

Chloroform 



. 23 

3-9 J 

Formic ether 



. 60-4 

21-3 ^ 

Formic ether 



. . 60'5 

21-3 J 

Sulphuric ether . 



. 62-3 

236 \ 

Sulphuric ether . 



. 62-5 

23-8 J 

Total heat. 



. 82-7 

100 


In the experiments recorded in the foregoing table, a total 
heat of 82*7°, or 588 units, was employed; and to test whether 
the absorption calculated from this high total agreed with the 













CONTRIBUTIONS TO MOLECULAR PHYSICS. 


221 


absorptions calculated from a low total, a portion of the current 
was diverted, the branch passing through the galvanometer 
producing a deflection of 49*4°. This corresponds to 77 units. 
The source of heat, it will be observed, is here quite unchanged; 
the rajs are of the same quality, and pass through the tube in the 
same quantity as before ; but in the one case the absorption is 
calculated from the deflection among the high degrees, and in 
the other case it is calculated from deflections among the low 
degrees of the galvanometer. 

The experiments were limited to formic and sulphuric ether, 
with the following results :— 






Deflection 

Absorption 
per 100 

Absorption from 
Table XIV. 

Formic ether 

• 

• 

• 

o 

. 177 

23 

21-3 

Sulphuric ether. 

• 

• 

• 

. 19-1 

24-8 

237 


The agreement is such as lo prove that no material error can 
have crept into the calibration. 

Placing the results obtained with the respective sources side 
by side, the influence of temperature on the transmission comes 
out in a very decided manner. 


Table XY .—Absorption of Heat by Vapours. 

Pressure, 0’5 of an inch. 

Source of heat: Platinum Spiral 


Name of Vapour 

Barely 

Bright 

White- 

Near 


visible 

red 

hot 

fusion 

Bisulphide of carbon 

. 6-5 

47 

2-9 

2-5 

Chloroform 

. 9-1 

6-3 

56 

39 

Iodide of methyl 

. 12-5 

9*6 

7-8 


Iodide of ethyl 

. 21 

177 

12-8 


Benzol .... 

. 26-3 

20-6 

16-5 


Amylene .... 

. 35-8 

27-5 

227 


Sulphuric ether 

. 434 

31-4 

25-9 

237 

Formic ether . 

. 45-2 

31-9 

25-1 

21*3 

Acetic ether 

. 49-6 

34-6 

272 



The gradual augmentation of penetrative power as the tem¬ 
perature is augmented is here very manifest. By raising the 
spiral from a barely visible heat to an intense white heat, we 
reduce the absorption, in the cases of bisulphide of carbon and 
chloroform, to less than one-half. At barely visible redness, 
moreover, 56*6 and 54*8 per 100 pass through sulphuric and 






’222 COXTRIBUTIOXS TO MOLECULAR PHYSICS. 

formic ether respectively; while, of the intensely white-hot 
spiral, 76-3 and 78*7 per 100 pass through the same vapours. 
By augmenting the temperature of solid platinum, we introduce 
into the radiation waves of shorter period, which, being in 
discord with the periods of the vapours, get more easily through 
them. 

What becomes of the more slowly recurrent vibrations as the 
more rapid ones are introduced ? Do the latter take the place 
of the former? This question is answered by experiments 
made with an opaque solution of iodine, and with lampblack. 
As the temperature of the platinum spiral increases from a 
dark heat to the most intense white heat, the absolute quantity 
transmitted through both these bodies steadily augments. But 
this heat is wholly obscure, for both the solution and the lamp¬ 
black intercept all the luminous heat. Hence the conclusion 
that the augmentation of temperature which introduces the 
shorter waves augments at the same time the amplitude of the 
longer ones, and hence also the inference that a body like the 
sun must of necessity include in its radiation waves of the same 
period as those emitted by obscure bodies. 

§ 9 . 

Changes of Diathermancy through Changes of Temperature .— 

Radiation from Lampblack at 100 c C. compared with that from 

white-hot Platinum. 

Running the eye along the numbers which express the 
absorptions of sulphuric and formic ether in Table XY., we 
find that, for the lowest heat, the absorption of the letter 
exceeds that of the former; for a bright red heat they are 
nearly equal, the formic still retaining a slight predominance; 
at a white heat, however, the sulphuric slips in advance, and 
at the heat near fusion its predominance is decided. I have 
tested this result in various ways, and by multiplied experiments, 
and placed it beyond doubt. We may at once infer from it 
that the capacity of the molecule of formic ether to enter into 
rapid vibration is less than that of sulphuric. By augmenting 
the temperature of the spiral we produce vibrations of quicker 
periods, and the more of these that are introduced, the 
more transparent, in comparison with sulphuric ether, does 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


223 


formic ether become. Thus its 6 complexity ’ tells upon the 
vibrating periods of the formic ether, the atom of oxygen 
which it possesses in excess of the sulphuric ether rendering 
it a more sluggish vibrator. Experiments made with a source 
of 212° Ealir. establish more decidedly the preponderance of 
the formic ether for slow vibrations. 


Table XVI.— Radiation through Vapours, 

Source of heat: Leslie's Cube , coated with Lampblack. 
Temperature, 212° Fahr. 


Name of Vapour 
Bisulphide of carbon 
Iodide of methyl . 
Chloroform . 
Sulphuric ether 
Formic ether 


Absorption per 100 
. 64 

. 184 

. 19-5 

. 54-8 

. 60-9 


For heat issuing from this source, the absorption of formic 
ether is 6T per cent, in excess of that of sulphuric. 

Deeming the result worthy of rigid confirmation, I once 
more determined the order of absorption:— 


Table XVII. 

Source of heat: Leslie's Cube , coated with Lampblack. 
Temperature, 212° Fahr. 


Name of Vapour Deflections 

o 


Bisulphide of carbon 

.9-3 

Iodide of methyl 

.25 

Chloroform . 

.26-5 

Sulphuric ether 

.47 3 } 

Sulphuric ether 

. 477) 

Formic ether 

. 49-7) 

Formic ether 

.49-9 \ 


When the absorptions were calculated from these deflections, 
that of formic ether was found to be 6*3 per cent, in excess ot 
that of sulphuric. In the last table the excess was 6T. 

But in both Tables XVI. and XVII. we notice another case 
of reversal. In all the experiments with the platinum spiral 
recorded in Table XV., chloroform showed itself less energetic 
as an absorber than iodide of methyl; but in Tables XVI. and 
XVII. chloroform proved to be decidedly the more powerful of 
the two. Cases of this kind have, in my estimation, a peculiar 
significance, and I therefore took care to verify them. Three 














224 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


different series of experiments with the vapours in question 
were therefore executed, with the following results : 

Table XYIII. —Radiation through Vapours. 

Source of heat: Blackened Cube of Boiling Water. 


Absorptions per 100 


Name of Vapour 


I. 

II. 

III. 

Bisulphide of carbon . 


. 6-4 

6-6 


Iodide of methyl 


. 18-4 

18-8 

18-3 

Chloroform 


. 19-5 

2U6 

20-6 

Sulphuric ether . 


. 54-8 

54-1 

53-2 

Formic ether 


. 60-9 

60-4 

60 


Were it essential to my purpose, I should certainly be able 
to cause even the small differences which here show themselves 
to disappear. But the agreement is such as to place the cor¬ 
rectness of the experiments beyond doubt. It ivill be seen that, 
contrary to the results obtained with a white-hot spiral, in all three 
cases, ivhere a blackened cube of boiling water was the source, 
chloroform exceeds iodide of methyl, and formic ether exceeds 
sulphuric in absorbent power. 

To clench the demonstration, I once more resorted to the 
white-hot spiral, and obtained the following results :— 

Table XIX. —Radiation through Vapours. 

Source of heat: White-hot Platinum Spiral. 


Name of Vapour 

Chloroform .... 



Deflection 

o 

. 9-8 

Absorption per 100 

4-5 

Chloroform 




. 9-5 

4-5 

Iodide of methyl . 




. 16 

7-3 

Iodide of methyl . . 




. 15-8 

73 

Formic ether . 




. 42-1 

24-2 

Formic ether . 




. 42-3 

24-5 

Sulphuric ether 




. 43-6 

26-3 

Sulphuric ether 




. 43-5 

26*2 

Total heat 




. 70-9 

100 


Here chloroform retreats once more behind iodide of methyl, 
and formic ether behind sulphuric. 

§ 10 . 

Changes of Diathermancy through Change of Source of Heat .— 
Radiation from Platinum and from Lampblack at the same 
Temperature. 

The positions of sulphuric and formic ether are reversed 
within the range of the experiments made with the platinum 












CONTRIBUTIONS TO MOLECULAR PHYSICS. 


225 


spiral, but this is not the case with the chloroform and the 
iodide of methyl. Even when the spiral was at a barely 
visible heat, the iodide was decidedly the most opaque of the 
two. The same result was obtained with a spiral heated below 
redness, as proved by the following figures:— 


Name of Vapour 





Deflection 

Absorption per 100 

Chloroform 





o 

. 8-5 

12-14 

Chloroform 





. 8'5 

12-14 

Iodide of methyl 





. 10 

14-28 

Iodide of methyl . 





. 10 

14-28 

Total heat 





. 47-3 

100 


Here the iodide is still predominant. Is it, then, a question 
of temperature merely ? or is there a special flux emitted by the 
lampblack, to which chloroform is particularly opaque ? In other 
words, is there a special accord between the rates of vibration 
of lampblack and chloroform? To answer this question I 
operated thus:—The platinum spiral was heated by only two 
cells, and the strength of this current was lowered by the 
introduction of resistance. When decidedly below a red heat, 
the spiral was plunged into boiling water. Bubbles of steam 
issued from it, proving that its temperature was above 212° 
Fahr. By augmenting the resistance its heat was lowered, 
until it was no longer competent to produce the least ebullition. 
It was then withdrawn from the water, and employed as a 
source : the following are the results :— 


Table XX. — Radiation through Vapours. 

Source of heat: Platinum Spiral at 100° C. 

Name of Vapour Deflection Absorption per 100 

o 

Bisulphide of carbon.5'7 7'03 

Chloroform.14 168 

Iodide of methyl.15*3 18 

No reversal was here obtained. The temperature was then 
reduced so that the total heat fell from 81 units to 59 units; 
but not even in this case (when the temperature was consider¬ 
ably below that of boiling water) could the reversal be obtained. 
The absorptions approach each other, but the iodide has still 
the advantage of the chloroform. Here are the numbers:— 

15 







226 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Table XXI .—Radiation through Vapours, 

Source of heat: Platinum Spiral, heated under 100° C. 


Name of Vapour 




Deflection 

Absorption per 100 

Bisulphide of carbon 

• 

• 

• 

o 

. 5-2 

9-2 

Chloroform 

• 

• 

• 

. 10 

17 3 

Iodide of methyl . 

• 

• 

• 

. 10-8 

18-2 


It is not, therefore, temperature alone which determines the 
inversion: the experiments prove that there is a greater 
synchronism between the vibrating periods of chloroform and 
lampblack than between those of chloroform and platinum 
raised to the temperature of the lampblack. It is seen, however, 
that as the temperature of the platinum falls, tjie opacity of 
the chloroform increases more quickly than that of the iodide: 
with an intensely white-hot spiral, as shown in Table XXI., 
the absorption of chloroform is to that of the iodide as 100 : 162, 
while with the spiral heated to a temperature of 212° Fahr., 
the ratio of the absorption is as 100 : 105. 

§ 11 . 

Radiation from Flames through Vapours.—Further Changes of 

Diathermancy. 

We have hitherto occupied ourselves with the radiation from 
heated solids : I will now pass on to the examination of the 
radiation from flames. The first experiments were made with 
a steady jet of gas issuing from a small circular burner, the 
flame being long and tapering. The top and bottom of the 
flame were excluded, and its most brilliant portion was chosen 
as the source of heat. The following results were obtained:— 

Table XXII.— Radiation of Heat through Vapours. 

Source of heat: A highly luminous Jet of Gas. 


Name of Vapour 

Deflection 

Absorption 
per 100 

White-hot 

Spiral 

Bisulphide of carbon 

o 

. 8-9 

9-8 

2*9 

Chloroform 

. 10*9 

12 

5G 

Iodide of methyl 

. . 15-4 

16-5 

7-8 

Iodide of ethyl . 

. 177 

19*5 

12-8 

Benzol 

. 20 

22 

16*5 

Amylene . 

. 275 

30*2 

22*7 

Formic ether 

. . 31-5 

34-6 

251 

Sulphuric ether . 

. 32-5 

35*7 

25*9 

Acetic ether 

. . 342 

387 

27-2 

Total heat , 

. 53-8 

100 






CONTRIBUTIONS TO MOLECULAR PHYSICS. 227 

To facilitate the comparison of the white-hot carbon with 
the white-hot platinum, I have here placed beside the results 
in the last table those recorded in Table XIII. The emission 
from the flame is thus proved to be far more powerfully absorbed 
than the emission from the spiral. Doubtless, however, the 
carbon, in reaching incandescence, passes through lower stages 
of temperature, and in those stages emits heat more in accord 
with the vapours. It is also mixed with the vapour of water 
and carbonic acid, both contributing their quota to the total 
radiation. It is therefore probable that the greater accord 
between the periods of the flame and those of the vapours is 
due to the slower periods of the substances which are unavoid- 
ably mixed with the incandescent carbon. 

The next source of heat employed was the flame of a Bunsen’s 
burner, the temperature of which is known to be very high. 
The flame was of a pale-blue colour, and emitted a very feeble 
light. The following results were obtained :— 


Table XXIII. Radiation of Seat through Vapours, 

Source of heat: Vale-blue Flame of Bunsen's Burner, 


Name of Vapour 

Deflection 

Absorption 
per 100 

From Table XXII. 
Luminous Jet of Gas 

Chloroform . 

o 

. 5 

6-2 

12 

Bisulphide of carbon . 

. 9 

111 

98 

Iodide of ethyl . 

. 113 

14 

19-5 

Benzol 

. 14-5 

17-9 

22 

Amylene 

. 19-6 

24-2 

30-2 

Sulphuric ether . 

. 25-8 

31-9 

357 

Formic ether 

. 27 

333 

34-6 

Acetic ether 

. 20-4 

36-3 

387 

Total heat . 

. 50-6 

100 

100 


^ Comparing Tables XXII. and XXIII., we see that the radia¬ 
tion from the Bunsen’s burner is, on the whole, less powerfully 
absorbed than that from the luminous gas jet. In some cases, 
as in that of formic ether, they come very close to each other; 
m the case of amylene and a few other substances they differ 
more markedly. But an extremely interesting case of reversal 
here shows itself. Bisulphide of carbon, instead of being first, 
stands decidedly below chloroform. With the luminous jet, 





228 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


the absorption of bisulphide of carbon is to that of chloroform 
as 100 : 122, while with the flame of Bunsen’s burner the ratio 
is 100 : 5G; the removal of the carbon from the flame more than 
doubles the relative transparency of the chloroform. The case is 
of too much interest to be passed over without verification. 
Here is the result obtained with a different total heat:— 


Source of beat: 

Pale-blue Flame of Bunsen's Burner. 

Deflection Absorption 

Chloroform . 

o 

.165 

8-4 

Chloroform 

.16 

8-2 

Bisulphide of carbon 

.19 

97 

Bisulphide of carbon 

.19-4 

99 

Total heat 

.68-4 

100 


And again, with an intermediate total heat:— 

Source of heat: Tcdc-blue Flame of Bunsen's Burner. 


Chloroform 
Chloroform 
Bisulphide of carbon 
Bisulphide of carbon 
Total heat 


Deflection 

o 

Absorption 

10-2 

8'4 

10 

8-4 

12 

9-8 

11-8 

9*7 

60 

100 


There is therefore no doubt that, while in the case of a pla¬ 
tinum spiral at all temperatures, of a luminous gas-flame, 
and, more especially, of lampblack heated to 212° Fahr. the 
absorption of chloroform exceeds that of bisulphide of carbon, 
for the flame of a Bunsen’s burner the bisulphide is the more 
powerful absorber of the two. The absorptive energy of the 
chloroform, as shown in Table XVIII.,. is more than three times 
that of the bisulphide, while in Table XXIII. the action of the 
bisulphide is nearly half as much again as that of the chloroform. 
We have here, moreover, another instance of the reversal of 
formic and sulphuric ether. For the luminous jet the sul¬ 
phuric ether is decidedly the more opaque; for the flame of 
Bunsen’s burner it is excelled in opacity by the formic. 

The total heat radiated from the flame of Bunsen’s burner is 
greatly less than that radiated when the incandescent carbon is 
present in the flame. The moment the air is permitted to mix 
with the luminous flame, the radiation falls so considerably that 
the diminution is at once detected, even by the hand or face 
brought near the flame. 












CONTRIBUTIONS TO MOLECULAR PHYSICS. 


229 


§ 12 . 

Radiation of Hydrogen Flame through Dry and Humid Air .— 
Influence of Vibrating Period on the Absorption. 

The main radiating bodies in the flame of a Bunsen’s burner 
are, no doubt, aqueous vapour and carbonic acid. Highly 
heated nitrogen is also present, which may produce a sensible 
effect: the unburnt gas, moreover, in proximity with the flame, 
and warmed by it, may contribute to the radiation, even before 
it unites with the atmospheric oxygen. But the main source of 
the radiation is, no doubt, the aqueous vapour and the carbonic 
acid. I wished to separate these two constituents, and to study 
them separately. The radiation of aqueous vapour could be 
obtained from a flame of pure hydrogen, while that of carbonic 
acid could be obtained from an ignited jet of carbonic oxide. 
To me the radiation from the hydrogen flame possessed a 
peculiar interest; for, notwithstanding the high temperature 
of such a flame, I thought it likely that the accord between its 
periods of vibration and those of the cool aqueous vapour of the 
atmosphere might be such as to cause the atmospheric vapour 
to exert a special absorbent power. The following experiments 
test this surmise :— 

Table XXIV. — Radiation through Atmospheric Air. 



Source of heat: A Hydrogen Flame. 




Deflection 

Absorption per 100 

Dry air . 


o 

. 0 

0 

Undried air 

• ••••< 

, . 21 - 5 

17-20 

Total heat 

• ••••< 

. 60-4 

100 


Thus, in a polished tube 4 feet long, the aqueous vapour of our 
laboratory air absorbed 17 per cent, of the radiation from the 
hydrogen flame. Of the radiation of a platinum spiral, heated 
by electricity to a degree of incandescence not greater than 
that obtainable by plunging a wire into the hydrogen flame, 
the undried air of the laboratory absorbed 

5'8 per cent., 

or one-third of the quantity absorbed when the flame of 
hydrogen was employed. 


230 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


The plunging of a spiral of platinum wire into the flame 
reduces its temperature; but it at the same time introduces 
vibrations which are not in accord with those of aqueous vapour : 
the absorption by ordinary undried air of heat emitted by this 
composite source amounted to 

8’6 per cent. 

On humid days the absorption of the rays emitted by a hy¬ 
drogen flame exceeds even the above large figure. Employing 
the same experimental tube and a new burner, the experiments 
were* repeated some days subsequently, with the following 
result:— 

Table XXV .—Radiation through Air. 

Source of heat: Hydrogen Flame. 

Absorption per 100 

Pry air.0 

Undried air.20'3 

Total heat.100 

The undried air here made use of embraced the carbonic acid 
of the atmosphere; the air was afterwards conducted through a 
tube containing a solution of caustic potash, in which the 
carbonic acid was intercepted, while the air charged itself 
with a little additional moisture. The absorption then observed 
amounted to 

20 3 per cent. 

of the entire radiation. The exact agreement of this with the 
last result is, of course, an accident; the additional humidity of 
the air derived from the solution of potash happened to com¬ 
pensate for the action of the carbonic acid withdrawn. 


§ 13. 

Radiation of Carbonic-oxide Flame through Dry and Humid Air , 
and through Carbonic Acid Gas.—Further illustration of Influ¬ 
ence of Vibrating Period. 

The other component of the flame of Bunsen^ burner is 
carbonic acid ; and the radiation of this substance is imme¬ 
diately obtained from a flame of carbonic oxide. With the air 
of the laboratory the following results were obtained:— 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


231 


Table XXYI .—Radiation through Atmospheric Air. 

Source of heat: Carbonic-oxide Flame (very small). 


Dry air . 






Deflection 

o 

Absorption per 100 

0 

Undried air . 

• 

• 

• 

• 

• 

. 10 

16-1 

Total heat 







100 


Of the heat emitted by carbonic acid, 16 per cent, was 
absorbed by the common air of the laboratory. After the air 
had been passed through sulphuric acid, the aqueous vapour 
being thus removed while the carbonic acid remained, the 
absorption was 

13-8 per cent. 

An india-rubber bag was filled from the lungs ; it contained 
therefore both the aqueous vapour and the carbonic acid of the 
breath. It was then conducted through a drying apparatus, 
the mixed air and carbonic acid being permitted to enter the 
experimental tube. The following results were obtained:— 


Table XXYII .—Air from the Lungs containing CO 2 . 

Source of heat: Carbonic-oxide Flame. 


Pressure in inches 

Deflection 

Absorption per 100 

1 

o 

7-2 

12 

3 

15 

25 

5 

20 

33-3 

30 

30-8 

50 

Total heat 

• • • r 

. 100 


Thus the tube filled with dry air from the lungs intercepted 
50 per cent, of the entire radiation from a carbonic-oxide flame. 
It is quite manifest that we have here a means of testing with 
surpassing delicacy the amount of carbonic acid emitted under 
various circumstances in the act of expiration.* 

That pure carbonic acid is highly opaque to the radiation from 
the carbonic-oxide flame, is forcibly evidenced by the results 
recorded in the following table :— 

* My late assistant, Mr. W. F. Barrett, subsequently carried out this notion. See 
article ‘ On a Physical Analysis of the Human Breath/ Philosophical Magazine , vol. 
xxviii. p. 108. 




232 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Table XXVIII.— Radiation through dry Carbonic Acid. 

Source of heat: Carbonic-oxide Flame. 


Pressure in inches 

Deflection 

o 

Absorption per 100 

1 

33-7 

53 

2 

37 

61-7 

3 

38-6 

66-9 

4 

39-4 

70 

6 

40 

72-3 

10 

41-4 

78-7 


About four months subsequent to the performance of these 
experiments they were repeated, using as a source of heat a 
much smaller flame of carbonic oxide. The absorptions were 
found somewhat less, but still very high. They follow in the 
next table. 

Table XXIX.— Radiation through dry Carbonic Acid. 

Source of heat: Small Carbonic-oxide Flame. 


Pressure in inches 

Deflection 


Absorption per 100 

1 

o 

17-3 


48 

2 * 

20 


555 

3 

21-7 


60-3 

4 

22-8 


65-1 

5 

24 


68-G 

10 

26 


74-3 


For the rays emanating from the heated solids employed in 
all my former researches, carbonic acid proved to be one of the 
most feeble of gaseous absorbers; but here, when the waves 
sent into it emanate from molecules of its own substance, its 
absorbent energy is enormous. The thirtieth of an atmosphere 
of the gas cuts off half the entire radiation ; while at a pressure 
of 4 inches, nearly 70 per cent, is intercepted. 


§14. 

Comparative Radiation of Carbonic-oxide Flame through Carbonic 

Acid Gas and Olefiant Gas. 

The energy of olefiant gas, both as an absorbent and a radiant, 
is well known to the reader of these memoirs; for the solid 
sources of heat just referred to, its power is incomparably 
greater, while for the radiation from the carbonic-oxide flame 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 233 

its power is far feebler than that of carbonic acid. This is 
proved by the following' experiments :— 


Table XXX. — Radiation through dry Olefiant Gas . 


Source of heat: Carbonic-oxide Flame. 


Pressure in inches Deflection 


1 

2 

4 

Total heat 


17 

26 

33 

47'3 


Absorption per 100 

24-2 

37'1 

49-1 

100 


Four months subsequent to the performance of the above 
experiments, a second series were made with olefiant gas, and 
the following results obtained :— 


Table XXXI. — Radiation through dry Olefiant Gas. 

Source of heat: Small Carbonic-oxide Flame. 


Pressure in inches 

Deflection 

Absorption per 100 

From Table! 

1 

o 

11*4 

23-2 

48 

2 

17 

347 

55o 

3 

21-6 

44. 

60-3 

4 

24-8 

50-6 

65’ 1 

5 

27 

55-1 

68-6 

10 

32-1 

. 65'5 

743 


Besides the absorption by olefiant gas, I have placed that by 
carbonic acid derived from Table XXIX. The superior power 
of the acid is most decided in the smaller pressures ; at a pressure 
of an inch it is twice that of the olefiant gas. The substances 
approach each other more closely as the quantity of gas aug¬ 
ments. Here, in fact, both of them approach perfect opacity; 
and as they draw near to this common limit, their absorptions, 
as a matter of course, approximate. 


§ 15 . 


Radiation of Hydrogen Flame through Carbonic Acid Gas and 

Olefiant Gas. 


A comparison of these results with the radiation of a hydro¬ 
gen flame through carbonic acid gas and olefiant gas respec¬ 
tively, brings out with great distinctness the differences of 
the radiant qualities of the two flames. 


234 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Table SXXII.— Radiation through Carbonic Acid Gas . 

Source of heat: Hydrogen Flame . 


Pressure in inches 

Deflection 

Absorption per 100 

1 

o 

5-5 

7-4 

* 2 

9-5 

12-8 

4 

11 

14*9 

30 

19 

257 

Total heat . 

. 48-5 

100 • 


Table XXXIlI.— Radiation through Olefiant Gas . 

Source of heat: Hydrogen Flame. 


Pressure in inches 

Deflection 

Absorption per 100 

From Table XXXII. 

1 

o 

12 

16-2 

7*4 

2 

18 

243 

12-8 

4 

24 

32-4 

14-9 

30 

38-5 

58-8 

257 

Total heat 

. 48-5 

100 

100 


A comparison of the last two columns, one of which is trans¬ 
ferred from Table XXXII., proves the absorption of the rays 
from a hydrogen flame by olefiant gas to be about twice that of 
carbonic acid; while, when the source of heat was a carbonic- 
oxide flame, the absorption by carbonic acid at small pres¬ 
sures was more than twice that of olefiant gas. 

The temperature of a hydrogen flame, as calculated by 
Bunsen, is 3259° C., while that of a carbonic-oxide flame is 
3042° C. The foregoing experiments demonstrate that accord 
subsists between the oscillating periods of these sources of heat 
and the periods of aqueous vapour and carbonic acid at a tem¬ 
perature of 15° C. The heat of these flames goes to augment 
the amplitude, and not to guicken the vibration. 


§ 16 . 


Radiation of Carbonic-oxide Flame through Carbonic Oxide , dn'd 
of Bisulphide-of-Carbon Flame through Sulphurous Acid . 

Sent through carbonic oxide, the radiation from the carbonic- 
oxide flame gave the following absorptions :— 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


235 


Table XXXIY.— Radiation through Carbonic Oxide. 

Source of heat: Carbonic-oxide Flame. 


Pressure in inches 

Deflection 

o 

Absorption per 100 

1 

18 

29 

2 

27 

43-5 

4 

34 

56-4 

10 

37*3 

655 


The absorptive energy is here high—higher, indeed, than 
that of olefiant gas; but it falls considerably short of that 
of carbonic acid. This result shows that the main radiant in 
the flame is its product of combustion, and not the carbonic 
oxide heated prior to combustion. 

To examine the radiation through sulphurous acid of a 
flame whose product of combustion is sulphurous acid, I re¬ 
sorted to the flame of bisulphide of carbon. Here, however, 
we had carbonic acid mixed with the sulphurous acid of the 
flame. Of the heat radiated by this composite source of heat, 
the absorption by an atmosphere of sulphurous acid amounted to 

60 per cent. 

The gas was sent from its generating retort through drying- 
tubes of sulphuric acid into a glass experimental tube 2*8 feet 
long. The comparative shortness of the tube, and the mixed 
character of the radiation, rendered the absorption less than 
it would have been had a source of heat of pure sulphurous 
acid and a tube as long as that used in the other experiments 
been employed. 


§ 17 - 

Radiation of the Flames of Carbonic Oxide and Hydrogen through 
Sulphuric and Formic Ether Vapours.—Reversal of Order of 
Absorption . 

To test the comparative penetrative powers of the two sources 
of heat I subsequently caused the radiation from the carbonic- 
oxide flame to pass through the vapours of formic and sulphuric 
ether at a common pressure of 0'5 of an inch with the 
following results :— 


236 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Table XXXV. 


Source of heat: Carbonic-oxide Flame. 


Formic ether 
Sulphuric ether 
Total heat . 



Deflection 

o 

Absorption per 100 

• 

14-5 

25-8 

• 

18 

32-1 

• 

43 

100 


Table XXXVI. 

Source of heat: Hydrogen Flame. 


Sulphuric ether 

• 

• 

Deflection 

o 

32 

Formic ether 

• 

• 

. 35 

Total heat . 

• 

• 

48-5 


Absorption per 100 

42-2 

49-3 

100 


We here find that, in the case of every one of the four vapours, 
the synchronism with hot aqueous vapour is greater than with 
hot carbonic acid. The temperature of the hydrogen flame is 
higher than that of the carbonic oxide ; but the radiation from 
the more intense source of heat, instead of possessing the greatest 
penetrative power, is the most copiously absorbed. It has been 
already proved that, for waves of slow period, formic ether is 
more absorbent than sulphuric ether; while for waves of rapid 
period, the sulphuric ether is the more powerful absorber. For 
the radiation from hot carbonic acid, the absorption of sulphuric 
ether, as shown in Table XXXV., is between 6 and 7 per cent, 
in excess of that of formic ether ; while for the radiation from 
hot aqueous vapour, the absorption of formic ether, as shown in 
Table XXXVI., is 7 per cent, in excess of that of sulphuric. 
That the periods of aqueous vapour, as compared with those of 
carbonic acid, are slow, and that .Jt is the aqueous vapour, and 
not the carbonic acid, of the flame of Bunsen’s burner which 
causes the reversal noticed in Table XXIII., may therefore be 
inferred from these experiments. 


• § IB. 

Radiation of Hydrogen Flame. and of Platinum Spiral plunged in 
Hydrogen Flame , through Liquids.—Conversion of Long Periods 
into Short ones. 

i 

Water at moderate thickness is a very transparent substance; 
that is to say, the periods of its molecules are in discord with 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


those of the visible spectrum. It is also highly transparent to 
the ultra-violet rays ; so that we may safely infer from the de¬ 
portment of this substance its incompetence to enter into rapid 
molecular vibration. When, however, we once quit the visible 
spectrum for the rays beyond the red, the opacity of the sub¬ 
stance begins to show itself: for such rays, indeed, its absorbent 
power is unequalled. The synchronism of the periods of the 
water-molecules with those of the ultra-red waves is thus 
demonstrated. 

. ' * 

The vibrating-period of a molecule is, no doubt, determined 

by the elastic forces which separate it from other molecules, and 
it is worth inquiring how these forces are affected when a 
change so great as that of the passage of a vapour to a liquid 
occurs. The fact established in the earlier sections of this 
paper, that the order of absorption for liquids and their vapours 
is the same, renders it extremely probable that the period of 
vibration is not materially affected by the change from vapour 
to liquid; for, if changed, it would probably be changed in dif¬ 
ferent degrees for the different liquids, and the order of 
absorption would be thereby disturbed.* The following table 
will throw additional light upon this question :— 


Table XXXVII .—Radiation through Liquids . 

Source of heat: Hydrogen Flame. 

Thickness of liquid layer, 0 - 07 of an inch. 


Name of liquid 

Absorption per 100 

Transmission 

Bisulphide of carbon 

277 

72-3 

Chloroform . 

49-3 

50-7 

Iodide of ethyl 

75-6 

24-4 

Benzol 

82-3 

17*7 

Amylene 

87-9 

12-1 

Sulphuric ether 

92-6 

7-4 

Formic ether 

935 

6*5 

Acetic ether . 

93*9 

6-1 

Water .... 

. 100 



Through a layer of water 9-21 millimetres thick, Melloni found 
a transmission of 11 per cent, of the heat of a Locatelli lamp. 

# q’JjQ general agreement in point of colour between a liquid and its vapour favours 
the idea that the period, at all events in the great majority of cases, remains constant 
when the state of aggregation is changed. 









238 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Here we employ a source of lieat of higher temperature, and a 
layer of water only one-fifth of the thickness used by Melloni, 
and still we find the whole of the heat intercepted.* A layer 
of water, 0'07 of an inch in thickness, is sensibly opaque to the 
radiation from a hydrogen flame. Hence we may infer the coin¬ 
cidence in period between cold water and aqueous vapour heated 
to a temperature of 3259° C.; and inasmuch as the period of 
the water-molecules has been proved to be ultra-red, the period 
of the vapour-molecules in the hydrogen flame must be ultra- 
red also. 

Another point of considerable interest may here be adverted 
to. Professor Stokes has demonstrated that a change of period 
is possible to those rays which belong to the violet and ultra¬ 
violet end of the spectrum, the change showing itself by a 
degradation of the refrangibility. That is to say, vibrations of 
a rapid period are absorbed, and the absorbing substance has 
become the source of vibrations of a longer period. Efforts, I 
believe, have been made to obtain an analogous result at the red 
end of the spectrum, but hitherto without result; and it has 
been considered improbable that a change of period can occur 
which should raise the refrangibility of the light or heat. 

Such a change, I believe, occurs when we plunge a platinum 
wire into a hydrogen flame. The platinum is rendered white 
by the collision of molecules whose periods of oscillation are 
incompetent to excite vision. There is, therefore, in this com¬ 
mon experiment an actual breaking up of the long periods into 
short ones—a true rendering of unvisual periods visual. The 
change of refrangibility differs from that of Professor Stokes, 
first, in its being in the opposite direction—that is, from low 
to high ; and secondly, in the circumstance that the platinum is 
heated by the collision of the molecules of aqueous vapour, and 
before their heat has assumed the radiant form. But it cannot 
be doubted that the same effect would be produced by radiant 
heat of the same period, provided the motion of the cether could 
be raised to a sufficient intensity. The effect in principle is 

* From the opacity of water to the radiation from aqueous vapour, we may infer 
the opacity of aqueous vapour to the radiation from water, and hence conclude that 
the very act of nocturnal refrigeration which causes the condensation of water on the 
earth’s surface gives to terrestrial radiation that particular character which renders it 
most liable to be intercepted by the aqueous vapour of the air. 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 239 

the same, whether we consider the platinum wire to be struck 
by a particle of aqueous vapour oscillating at a certain rate, or 
bJ a particle of aether oscillating at the same rate. And thus, I 
imagine, by a chain of rigid reasoning, we arrive at the con¬ 
clusion that a degree of incandescence, equal to that of the sun 
itself, might be produced by the impact of waves of themselves 
incompetent to excite vision. 

The change of quality produced in the radiation by the intro¬ 
duction of a platinum spiral into a hydrogen flame is illustrated 
by a series of experiments, executed for me by my assistant, 
Mr. Barrett, and inserted subsequently to the presentation of 
this memoir. 


Table XXXYIII.— Radiation through Liquids. 

Sources of heat: 1. Hydrogen Flame; 

2. Hydrogen Flame and Platinum Spiral. 

Transmission 

---A- 

Thickness of Liquid Thickness of Liquid 

0-04 inch 0-07 inch 


Name of liquid 

Flame 

only 

Flame and 
spiral 

Flame 

only 

Flame and 
spiral 

Bisulphide of carbon. 

777 

87-2 

70-4 

86 

Chloroform 

54 

72-8 

507 

69 

Iodide of methyl 

31-6 

42*4 

26-2 

36-2 

Iodide of ethyl. 

30*3 

36-8 

24-2 

32-0 

Benzol 

24-1 

32-6 

17-9 

OO 

ob 

Amylene . 

. 14-9 

25-8 

12*4 

24-3 

Sulphuric ether 

13-1 

22-6 

8-1 

22 

Acetic ether 

i0-l 

1 8*3 

6-6 

18-5 

Alcohol . 

9-4 

14-7 

5-8 

12-3 

Water 

3-2 

7*5 

2 

6*4 


Here the introduction of the platinum spiral changed the 
periods of the flame into others more in discord with the periods 
of the liquid molecules, and hence the more copious transmission 
when the spiral was employed. It will be seen that a transmis¬ 
sion of 2 per cent, is here obtained through a layer of water 
0*07 of an inch in thickness, while in Table XXVII. all was 
absorbed. 

To test this point further, another series of experiments was 
executed, and gave the following results for the radiation of a 
hydrogen flame through layers of water of five different thick¬ 


nesses :— 













240 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


Radiation through Water . 

Source of heat: Hydrogen Flame. 

Thickness of liquid 

t ■ . ---\ 

0-02 0*04 0-07 0T4 0*27 

inch inch inch inch inch 

Transmission per 100 . . 5‘8 2'8 11 0*5 0*0 


§ 19. 

Radiation of Small Gas Flame compared with that of Hydrogen 
Flame.—Further Changes of Diathermic Position. 

Wishing to compare the radiation from a flame of ordinary 
coal-gas with that of onr hydrogen flame, I reduced the former 
to the dimensions of the latter. The flame thus diminished had 
a blue base and bright top, and the whole of it was permitted to 
radiate through our series of liquids. The following results 
were obtained:— 


Table XXXIX.— Radiation through Liquids. 

Source of heat: Small Gas Flame. 

Thickness of liquid, layer 0’07 of an inch. 


Name of Liquid 

Deflection 

Absorption 
per 100 

From 

Table XXXVII. 

Chloroform 

287 

• 39 8 

493 

Bisulphide of carbon 

36 

53-2 

27-7 

Iodide of ethyl 

41*7 

72-3 

75-6 

Benzol 

43-4 

79*4 

82-3 

Amylene. . . 

Sulphuric ether 

Formic ether . . 

45 

46-6 

46-6 

86-1 

93-3 

93-3 

87-9 
• 92-6 

93-5 

Alcohol . 

46-8 

941 


Acetic ether . 

46 9 

94-4 

93-9 

Water 

47-4 

97-1 

100 

Total heat 

48 

100 



I have placed the results obtained with the hydrogen flame 
in the third column of figures. It will be observed that the' 
absorption of the heat issuing from the small gas flame is, in 
some cases, nearly the same as that of the heat issuing from the 
flame of hydrogen. A very remarkable difference, however, 
shows itself in the deportment of bisulphide of carbon as com¬ 
pared with that of chloroform. For the small gas flame chloro¬ 
form is the most transparent body in the list; it is markedly 
more transparent than the bisulphide of carbon, while for the 






CONTRIBUTIONS TO MOLECULAR PHYSICS. 241 

hydrogen flame the bisulphide greatly excels the chloroform in 
transparency. The large luminous gas flame previously experi¬ 
mented with differs also from the small one here employed. 

With the large flame, the absorption by the bisulphide is to that 
by the chloroform as 

100 : 121 , 

while with the small flame the absorptions of the same two 
substances stand to each other in the ratio of 

100 : 76 . 

Numerous experiments were subsequently made, with a view of 

testing this result, but in all cases the bisulphide was found •. 

more opaque than the chloroform to the radiation of the small 

gas flame. The same result was obtained when a very small oil 

flame was employed ; and it came out in a very decided manner 

when the source of heat was a flame of bisulphide of carbon. 

It was found, moreover, that, whenever two liquids underwent a 
change of position of this hind, the vapours of the liquids underwent 
a similar change j in its finest gradations the deportment of the 
liquid was imitated by that of its vapour. 

§ 20 . 

Explanation of Certain Results of Melloni and Knoblauch. 

And here we find ourselves in a position to offer solutions of 
various facts which have hitherto stood as enigmas in researches 
upon radiant heat. It was for a long time supposed that 
the power of heat to penetrate diathermic substances aug¬ 
mented with the temperature of the source of heat, and from 
the exceptional penetrative power of solar heat inferences were 
drawn as to the enormous temperature of the sun. Knoblauch 
contended against this notion, showing that the heat emitted 
by a platinum wire plunged into an alcohol flame was less 
absorbed by certain diathermic screens than the heat of the 
flame itself, and justly arguing that the temperature of the 
spiral could not be higher than that of the body from which it 
derived its heat. A plate of glass being introduced between his 
source of heat and his thermo-electric pile, the deflection of 
his needle fell, from 35° to 19° when the source of heat was the 
platinum spiral; while, when the source of heat was the flame 

of alcohol, the introduction of the same glass caused the deflec- 
16 


242 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


tion to fall from 35° to 16°, proving that the radiation from the 
flame was intercepted more powerfully than that from the spiral 
—showing, in other words, that the heat emanating from the 
body of highest temperature possessed the least penetrative 
power. Melloni afterwards corroborated this experiment. 

Transparent glass allows the rays of the visible spectrum to 
pass freely through it; but it is well known to be highly opaque 
to the radiation from obscure sources of heat—in other words, 
to waves of long period. A plate 2*6 millimetres thick inter¬ 
cepts all the rays from a source of heat of 100° C., and transmits 
only 6 per cent, of the heat emitted by copper raised to 400° C.* 
Now the products of the combustion of alcohol are aqueous 
vapour and carbonic acid, whose waves have just been proved 
to be of slow period, or of the particular character most power¬ 
fully intercepted by glass. But by plunging a platinum wire 
into such a flame, we virtually convert its heat into heat of 
higher refrangibility; we break up the long periods into shorter 
ones, and thus establish the discord between the periods of the 
source of heat and the periods of the diathermic glass, which, 
as before defined, is the physical cause of transparency. On 
purely a priori grounds, therefore, we might infer that the 
introduction of the platinum spiral would augment the pene¬ 
trative power of the heat through the glass. With two plates 
of glass, of different thicknesses, Melloni found the following 
transmissions for the flame and the spiral:— 

For the flame For the platinum 

41*2 52'8 

57 20-2 

The same remarks apply to the transparent selenite examined 
by Melloni. This substance is highly opaque to the ultra-red 
undulations; but the radiation from an alcohol flame is almost 
wholly of this character, and hence the opacity of the selenite 
to this radiation. The introduction of the platinum spiral 
shortens the periods and increases the transmission. Thus, with 
two specimens of selenite, of different thicknesses, Melloni found 
the transmissions to be as follows :— 


Flame 

4-4 

1-7 


* Melloni. 


Platinum 

19-5 

3*5 



CONTRIBUTIONS TO MOLECULAR PHYSICS. 243 

So far tlie results of Melloni correspond with those of Knob¬ 
lauch; but the Italian philosopher pursues the matter further, 
and shows that Knoblauch’s results, though true for the par¬ 
ticular substances examined by him, are far from being appli¬ 
cable to diathermic media generally. In th,e case of black glass 
and black mica, a striking inversion of the effect is observed ; 
by these substances the radiation from the flame is more 
copiously transmitted than the radiation from the platinum. 
For two pieces of black glass, of different thicknesses, Melloni 
found the following transmissions :— 

From the flame From the platinum 

52-6 42:8 

29-9 27-1 

And for two plates of black mica the following transmissions :_ 

From the flame From the platinum 

62-8 52o 

43-3 28-9 

These results were left unexplained by Melloni; but the solution 
is now easy. The black glass and the black mica owe their 
blackness to the carbon incorporated in them, and the blackness 
of this substance, as already remarked, proves the accord of its 
vibrating-periods with those of the visible spectrum. But it 
has been proved that carbon is in a considerable degree pervious 
to the waves of long period—that is to shy, to those emitted 
by a flame of alcohol. The case of the carbon is therefore 
precisely antithetical to that of the transparent glass—the 
former transmitting the heat of long period and the latter 
the heat of short period most freely. Hence # it follows 
that the introduction of the platinum wire, by converting* 
the long periods of the flame into short ones, augments 
the transmission through the transparent glass and selenite, 
and diminishes it through the black glass and the black mica. 

§ 21 . 

Radiation of Hydrogen Flame through Lampblack, Iodine , and 
Rock-salt.—Diathermancy of Rock-salt examined. 

Lampblack, as already stated, is in accord with the undu¬ 
lations of the visible spectrum; it absorbs them all; but it is 
partially transparent to the waves of slow period. As, therefore. 


244 COXTRIBUTIOXS TO MOLECULAR PHYSICS. 

the waves issuing from a flame of hydrogen have been proved to 
be of slow period, we inay with probability infer that its radia¬ 
tion will penetrate the lampblack. A plate of rock-salt was 
placed over an oil-lamp until the layer of soot deposited on it 
was sufficient to intercept the light of the brightest gas-flame. 
The smoked plate was introduced in the path of the rays from 
the hydrogen flame, and its absorption was measured 5 the plate 
was then cleansed, and its absorption again determined. The 
difference of both gave the absorption of the layer of lampblack. 
The results were as follows :— 

Table XL. 

Deflection 

' o 

Smoked rock-salt .... 442 

Unsmoked plate .... 15‘8 

The difference between these gives us the 
lampblack ; it is 58-7 per cent. ; and this 
transmission of 

4U3 per cent. 

of the radiation from the hydrogen flame. 

Iodine, in a solution sufficiently opaque to cut off the light of 
our most brilliant l^inps, transmitted of the heat of the hydro¬ 
gen flame 

99 per cent. 

In experimenting on liquids with heat of slow period, it was 
noticed that the introduction of the empty rock-salt cell caused 
the needle to move through a much larger arc than when the 
source of heat was a luminous one. This suggested that a greater 
proportion of the heat of slow period was absorbed by the rock- 
salt. A few experiments were made to test the diathermancy of 
the salt, with the following results :— 

For the heat of a hydrogen flame, the transmission through 
a perfectly transparent plate of rock-salt was 

82’3 per cent. 

For a spiral of platinum wire heated to whiteness by an electric 
current, the transmission was 


Absorption per 100 

82-7 

24 

absorption of the 
corresponds to a 


87 per cent. 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 245 

For the same spiral lowered to bright redness, the transmission 
was , 

84*4 per cent. 

For the same spiral lowered to moderate redness, the transmis¬ 
sion was 

83'6 per cent. 

Nothing was changed in these experiments*but the heat of the 
spiral; the direction of the rays, and the size of the radiating 
body, remained throughout the same; still we find a gradually 
augmenting opacity on the part of the rock-salt as the tempera¬ 
ture of the source of heat is lowered. There cannot, I think, 
be a doubt that MM. De la Provostaye and Desains are right in 
their conclusion that rock-salt acts differently on different calo¬ 
rific rays, and is not, as Melloni supposed, equally transparent 
to all. For the heat of the hydrogen flame, moreover, it is more 
opaque than for that of the moderately red spiral. 


§ 22 . 


Physical Connexion between Radiation and Conduction . 


This memoir ought perhaps to end here. I would, however, 
ask permission to make a few additional remarks on a subject 
which was briefly touched upon towards the conclusion of the 
first of this series of memoirs. These remarks are made with 
diffidence, for I have reason to know that authorities worthy of 
the highest respect do not share my views regarding the 
connexion which subsists between the radiation and conduction 
ofjieat. 

Let us suppose heat to be communicated to a superficial 
stratum of the molecules of any body; say, those at the ex¬ 
tremity of a metal bar. They vibrate, and the motion com¬ 
municated by them to the external aether is despatched in waves 
through space. But motion must also be imparted to the 
aether within the body, and a portion of this motion will be 
transferred to the adjacent stratum of molecules, heat as a con¬ 
sequence appearing to penetrate the mass. But irrespective of 
the aether, the molecules occupy positions determined by their 
own attractive and repulsive forces ; so that if any one mole¬ 
cule be disturbed, it will of necessity disturb its neighbours. 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


In an aggregate of molecules so related, motion would be 
transmitted independently of the seth^r. If, indeed, we could 
imagine the aether entirely away, the motion that we call heat 
would still be propagated from molecule to molecule. In other 
words, conduction would manifest itself, while radiation would 
be absent through want of a medium. 

In matter, however, as we know it, molecular motion is only 
in part transmitted immediately from molecule to molecule, 
being more or less transmitted mediately by the aether. Now 
in the case just supposed, the quantity of motion transmitted by 
the internal aether to our second stratum of molecules cannot 
be the whole of that imparted to it by the superficial stratum. 
The aether must, to some extent, squander externally the 
internal molecular motion; so that were the medium absent 
—were the cushion removed which interferes with the direct 
propagation of motion from molecule to molecule—conduction 
would be freer than at present; the heat, suffering no lateral 
loss, would penetrate further into the mass than when the aether 
intervenes. 

This reasoning leads to the inference that those molecules 
which yield their motion most freely to the aether must on that 
account be the most wasteful as regards conduction; in other 
words, that the best radiators ought to prove themselves the 
worst conductors. 

A broad consideration of the subject shows this conclusion to 
be in general harmony with observed facts. Organic sub¬ 
stances are all exceedingly imperfect conductors of heat, and 
they are all excellent radiators. The moment, moreover, we 
pass from the metals to their compounds we pass from gpod 
conductors to bad ones, and from bad radiators to good ones.* 


* And we also pass, as a general rule, from a series of bodies which vibrate in ac¬ 
cord with the visible spectrum to a’ series which vibrate in discord with the spectrum. 
The lowering of the rate of vibration is a consequence of chemical union. The com¬ 
parative incompetence of compound bodies to oscillate in visual periods has incessantly 
declared itself in these researches. I would here refer to a most interesting illustra¬ 
tion of the same kind, derived from the experiments of MM. De la Provostaye and 
Desains. These distinguished experimenters were the first to record the important 
fact that the qualities of heat emitted by bodies at the same temperature may be very* 
unlike. Two experiments illustrate this. The first is recorded in the Comptes 
Ecndus, vol. xxxiv. p. 951. One half of a cube was coated with lampblack, and the 
other half with cinnabar. The cube being filled with oil at a temperature of 173° C., 
it was found that the emission from the cinnabar was more copiously absorbed by a 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


247 


From tlie earlier memoirs of MM. de la Provostaye and 
Desains,* and in that of MM. Wiedemann and Franz, I cull the 
following facts :—The radiative power of platinum is five times 
that of silver; its conductive power is one-tenth that of silver. 
Platinum has more than twice the radiative power of gold; it 
has only one-seventh of the conducting power. Zinc and tin are 
almost equal as conductors, and they are also nearly equal as 
radiators. Silver has about six times the conductive power of 
zinc and tin; it has only one-fourth of their radiative power. 
Brass possesses but one-half the radiative energy of platinum; it 
possesses more than twice its conductivity. Other experiments 
of MM. de la Provostaye and Desainsf confirm those first 
referred to. Taking the absorbent power, as determined by 
these excellent experimenters, to express the radiating power 
which will be allowed, and multiplying their results by a com¬ 
mon factor to facilitate comparison with those of MM. Wiede¬ 
mann and Franz on conduction, we obtain the following table:— 


Table XLII .—Comparison of Conduction and Radiation . 


r ame of metal 



Conduction 

Silver 



; 100 

Gold . 



53 

Brass. 



24 

Tin . 



15 

Platinum . 



8 


Radiation 

11 

27 

42 

90 

100 


We here find that, as the power of conduction diminishes, 
the power of radiation augments—a result, I think, completely 


plate of glass than that from the lampblack. In the second experiment, they found 
that, while 39 per cent, of the radiation from a bright surface of platinum was trans¬ 
mitted by a plate of glass, only 29 per cent, of the radiation from the opposite surface 
of the same plate, which was coated with borate of lead, w r as transmitted. These 
results are quite in harmony with the views which I have ventured to enunciate. We 
may infer from them that the heat emitted by the respective compounds— the cinnabar 
and the borate of lead—is of slower period than that emitted by the elements', for 
experiment proves that as the periods are quickened the glass becomes more trans¬ 
parent. At a temperature of 100° C., moreover, the emission from borate of lead was 
found equal to that from lampblack (Comptes Rendus, vol. xxxviii. p. ,442), while at a 
temperature of 550° C. it had only three-fourths of the emissive pow r er of the lamp¬ 
black. With reference to the theoretic view r s which these researches are intended to 
foreshadow, the results of MM. De la Provostaye and Desains are of the highest 
interest. 

* Comptcs Rendus , 184G, vol. xxii. p. 1139. 
f Annales de Chimie, 1850, vol. xxx. p. 442. 









248 


CONTRIBUTIONS TO MOLECULAR PHYSICS. 


in harmony -with that to which a consideration of the molecular 
mechanism leads us. 

There is hut one serious exception known to me to the law 
here indicated; this is copper, which MM. de la Provostaye 
and Desains place higher than gold as a radiator, though it is 
also higher as a conductor. When, however, the immense 
change in radiative power which the slightest film of an oxide 
can produce, and the liability of heated copper to contract such 
a film, are taken into account, the apparent exception will not 
have too much weight ascribed to it. I have had a cube of 
brass coated electrolytically with copper, silver, and gold; and, 
of all its faces, that coated with copper has the least emissive 
power. This is probably due to some slight impurity contracted 
by the silver. What we know of the deportment of minerals 
also illustrates the law. Rock-salt I find to be a far better con¬ 
ductor than glass, while MM. de la Provostaye and Desains 
find the relative emissive powers of the two substances to be as 
1 / to 6. So also with regard to alum: as a conductor it is 
immensely behind rock-salt; as a radiator it is immensely in 
advance of it. 


Royal Institution, March 1864 . 


YII 


ON LUMINOUS AND OBSCURE RADIATION. 


ANALYSIS OF MEMOIR VII. 




In the foregoing investigation, the conclusion had been reasoned out that the 
quality of the heat radiated by a flame of hydrogen was almost exclusively 
ultra-red, and the cha,nge produced by the plunging of solid bodies into the 
flame was pronounced to be a virtual exaltation of refrangibility. These con¬ 
clusions it was important to verify, and accordingly, the necessary rock-salt 
lenses and prisms having been secured, the emission from the hydrogen flame 
was subjected in 1864 to strict analysis. 

By direct experiment the reasoning was verified, and the emission was 
proved to be sensibly ultra-red. The rays of greatest energy of the hydrogen 
flame were proved to be of precisely the same refrangibility as the rays of 
greatest energy from a luminous gas-flame. 

The other conclusions enunciated in Memoir VI. regarding the raising 1 of 
solid bodies to incandescence by a hydrogen or an oxyhydrogen flame were also 
experimentally established. 

Intent on clearly bringing out the differences between elementary and com- ‘ 
poimd bodies in relation to radiant heat, I tried at an early period of these 
researches to extend the experiments to solids and liquids. From the physico¬ 
chemical point of view, the deportment of lampblack already revealed by 
Melloni was to me of peculiar interest and significance. But the interest was 
greatly augmented by the deportment of bromine and iodine. With various 
sources of heat the diathermancy of these two substances was illustrated. 
Leslie’s cubes containing boiling water, copper balls heated to various degrees 
of incandescence, gas and candle flames, were respectively examined, the sur¬ 
prising transparency of bromine and iodine to the calorific rays being in all 
cases demonstrated. 

The step from these experiments to sifting or filtering the radiation from 
luminous sources, by quenching the light and permitting the heat to pass, was 
inevitable and indeed immediate. After numerous experiments in the labora¬ 
tory of the Royal Institution, the filtering of the electric lamp and the forma¬ 
tion of powerful dark foci by the heat-rays emitted from the carbon-points, 
were publicly illustrated in the theatre of the Institution on March 27, 1862. 
The experiments are referred to in my ‘ Notes on Ileat,’ published at the time. 
They are also mentioned in a foot-note bearing date June 13, 1862, at the 
bottom of page 79 of this collection. 

The discussion with Professor Magnus being, as I thought, finally closed by 
the experimental evidence brought forward in Memoir VI., and I being still 
further assured by that investigation of the surprising diathermancy of iodine, 
the filtration of the emission from incandescent bodies became the subject of 
special investigation. 

The augmentation the energy of the invisible heat-rays by the increase of 
temperature necessary^to produce the visible ones, is determined; in the first 




ANALYSIS OF MEMOIR VII. 


251 


instance, by placing tbe pile in the ultra-red emission from a platinum spiral, as 
it rose gradually from a dark heat to an intense white one, a rock-salt prism 
being used to decompose the beam ; in a second instance by causing the spiral 
to pass through the same range of temperature, and cutting off its luminous 
rays by the iodine filter. 

The experiments are then extended to flames of coal-gas and to the electric- 
light. 

It is thus found that of the radiation from platinum heated to whiteness, 
one twenty-fourth only consists of luminous rays. 

Of the emission from the most brilliant portion of a gas flame, one twenty- 
fifth only consists of luminous rays. 

Of the emission from a dazzling electric-light, one-tenth only consists of 
luminous rays. 

Iodine is found to be perfectly transparent to the emission of bodies at all 
temperatures under incandescence. 

With the rock-salt lens and the iodine filter, the invisible rays of the electric- 
light are afterwards so concentrated as to ignite combustible bodies placed at the 
focus. 

* 

The eye is proved capable of bearing without inconvenience the heat of a 
focus where paper and other combustible bodies are ignited and gunpowder 
was exploded. 

Employing greater battery power, precisely the same effects are produced 
with the glass lenses used in 1862 to concentrate the invisible rays. 

From experiments on water, and on the vitreous humour of an ox, it is con¬ 
cluded that nearly two-thirds of the rays from the electric-light, which actually 
reach the retina, are obscure. 

It is further shown that the visible radiation from a red-hot platinum spiral 
is incapable of thermal measurement. 

The paper winds up with some remarks on the relation of light to heat, and 
on the application of radiant heat to fog-signalling. 













































































VII. 

ON LUMINOUS AND OBSCURE RADIATION.* * * § 


§ 1 . 

Spectrum of Hydrogen Flame. 

Sir William Herschel discovered tlie obscure rajs of the 
sun, and proved the position of maximum heat to be 
beyond the red of the solar spectrum.f Forty years subse¬ 
quently Sir John Herschel succeeded in obtaining a thermo¬ 
graph of the calorific spectrum, and in giving striking visible 
evidence of its extension beyond the red. J Melloni proved that 
an exceedingly large proportion of the emission from a flame 
of oil, of alcohol, and from incandescent platinum heated by a 
flame of alcohol, is obscure. § Dr. Miller inferred from its 
paucity of luminous rays evident to the eye, and a like paucity 
of ultra-violet rays, that the radiation from a flame of hydrogen 
must be mainly ultra-red; and he concluded from this that the 
glowing of a platinum wire in a hydrogen flame, as also the 
brightness of the Drummond light in the oxyhydrogen flame, 
are produced by a change in the period of vibration. |] By a 
different mode of reasoning I arrived at the same conclusion 
myself, and published the conclusion subsequently.^ 

A direct experimental demonstration of the character of the 
radiation from a hydrogen flame was, however, wanting, and 
this want I have sought to supply. I had constructed for me, 
by Mr. Becker, a complete rock-salt train of a size sufficient to 
permit of its being substituted for the ordinary glass train of a 

* Philosophical Magazine for November 1864. 

f Phil. Trans. 1800. 

J Phil. Trans. 1840. I hope very soon to be able to turn my attention to the 
remarkable results described in note III. of Sir J. Herschel’s paper. 

§ La Thermochrose, p. 304. • || Phil. Trans, vol. cliv. p. 327. 

*[ Report of the British Association, 1863. 


254 * 


LUMINOUS AND OBSCUEE RADIATION. 


Duboscq’s electric lamp. A double rock-salt lens placed 
in the camera rendered the rays parallel; the rays passed 
through a slit, and a second rock-salt lens placed without the 
camera produced, at an appropriate distance, an image of this 
slit. Behind this lens was placed a rock-salt prism, while 
laterally stood a thermo-electric pile intended to examine the 
spectrum produced by the prism. Within the camera of the 
electric lamp was placed a small burner, so that the flame 
issuing from it occupied the position usually taken wp by the 
coal points. This burner was connected with a T-piece, 
from which two pieces of india-rubber tubing were carried, the 
one to a large hydrogen-holder, the other to the gas-pipe of the 
laboratory. It was thus in my power to have, at will, either 
the gas flame or the hydrogen flame. When the former was 
employed, it produced a visible spectrum, which enabled me to 
fix the thermo-electric pile in its proper position. To obtain the 
hydrogen flame, it was only necessary to turn on the hydrogen 
until it reached the gas flame and was ignited; then to turn off 
the gas and leave the hydrogen flame behind. In this way the 
one flame could be substituted for the other without opening 
the door of the camera, or producing any change in the positions 
of the source of heat, the lenses, the prism, and the pile. 

The thermo-electric pile employed is a beautiful instrument 
constructed by Ruhmkorff. It belongs to my friend Mr. Gassiot, 
and consists of a single row of elements properly mounted and 
attached to a double brass screen. It has in front two silvered 
edges, which, by means of a screw, can be caused to close upon 
the pile so as to render its face as narrow as desirable, reducing 
it to the width of the finest hair, or, indeed, shutting it off 
altogether. By means of a small handle and long screw, the 
plate of brass and the pile attached to it can be moved gently 
to and fro, and thus the vertical slit of the pile can be caused 
to traverse the entire spectrum, or to pass beyond it in both 
directions. The width of the spectrum was in each case equal 
to the length of the face of the pile, which was connected with 
an extremely delicate galvanometer. 

I began with a luminous gas flame. The spectrum being 
cast upon the brass screen (which, to render the colours more 
visible, was covered with tinfoil), the pile was gradually moved 
in the direction from blue to red, until the deflection of the gal- 


LUMINOUS AND OBSCURE RADIATION. 


255 


variometer became a maximum. To reach this it was necessary 
to pass entirely through the spectrum and a little way beyond 
the red; the deflection then observed was 

30°. 

When the pile was moved in either direction from this position, 
the deflection diminished. 

The hydrogen flame was now substituted for the gas flame; 

the visible spectrum disappeared, and the deflection fell to 

\ 

12 °. 

Hence, as regards rays of this particular refrangibility, the 
emission from the luminous gas flame was two and a-half times 
that from the hydrogen flame. 

The pile was now moved to and fro, and the movement in 
both directions was accompanied by a diminished deflection. 
Twelve degrees, therefore, was the maximum deflection for the 
hydrogen flame; and the position of the pile, determined pre¬ 
viously by means of the luminous flame, proves that this 
deflection was produced by ultra-red undulations. I moved the 
pile a little forwards, so as to reduce the deflection from 12° to 
4°, and then, in order to ascertain the refrangibility of the rays 
which produced this small deflection, I relighted the gas. The 
rectilinear face of the pile was found invading the red. When 
the pile was caused to pass successively through positions 
corresponding to the various colours of the spectrum, and to 
its ultra-violet rays, no measurable deflection was produced by 
the hydrogen flame. 

I next placed the pile at some distance from the invisible 
spectrum of the flame of hydrogen, and felt for the spectrum by 
moving the pile to and fro. Having found it, the place of maxi¬ 
mum heating was without difficulty ascertained. Changing 
nothing else, the luminous flame was substituted for the non- 
luminous one; the position of the pile when thus revealed was 
beyond the red. 

The action was still very sensible when the distance of the 
pile from the red end of the spectrum on the one side was as 
great as that of the violet rays on the other, the heat-spectrum 
thus proving itself to be at least as long as the light-spectrum. 

It is thus proved experimentally that the radiation from a 
hydrogen flame is sensibly ultra-red. The other constituents of 






256 


LUMINOUS AND OBSCURE RADIATION. 


tlic radiation, are so feeble as to be thermally insensible. Hence, 
when a body is raised to incandescence by a hydrogen flame, the 
vibrating periods of its atoms must be shorter than those to which 
the radiation of the flame itself is due. 

§ 2 . 

Influence of Solid Particles. # 

The falling- of the deflection from 30° to 12° when the hydro¬ 
gen flame was substituted for the gas flame is doubtless due to 
the absence of all solid matter in the former. We may, how¬ 
ever, introduce such matter, and thus make a radiation 
originating in the hydrogen flame much greater than that of 
the gas flame. A spiral of platinum wire plunged in the former 
gave a maximum deflection of. 

52° 

at a time when the maximum deflection of the gas-flame was only 

33°. 

It is mainly by convection that the hydrogen flame disperses 
its heat: though its temperature is higher, its sparsely- 
scattered molecules are not able to cope, in radiant energy, 
with the solid carbon of the luminous flame. The same is true 
for the flame of a Bunsen’s burner; the moment the air (which 
destroys the solid carbon particles) mingles with the gas flame, 
the radiation falls considerably. Conversely, a gush of radiant 
heat accompanies the shutting out of the air which • deprives 
the gas flame of its luminosity. When, therefore, we introduce 
a platinum wire into a hydrogen flame, or carbon particles into a 
Bunsen's flame, we obtain not only waves of a new period, but also 
convert a large portion of the heat of convection into the heat o* 
radiation. 



Persistence and Strengthening of Obscure Rays by Augmentation 

of Temperature. 


Bunsen #nd Kirchhoff have proved that, for incandescent 
metallic vapours, the period of vibration is, within wide limits, 
independent of temperature. My own experiments with flames 


LUMINOUS AND OBSCURE RADIATION. 257 

of hydrogen and carbonic oxide as sources of heat, and with cold 
aqueous vapour and cold carbonic acid as absorbing media, point 
to the same conclusion.* But in solid metals augmented tem¬ 
perature introduces waves of shorter periods into the radiation. 

may be asked, ‘ What becomes of the long obscure periods 
when we heighten the temperature ? Are they broken up or 
changed into shorter ones, or do they maintain themselves side 

by side with the new vibrations?’ The question is worth an 
experimental answer. 

A spiral of platinum wire, suitably supported, was placed 
within the camera of the electric lamp at the place usually 
occupied by the carbon points. This spiral was connected with 
a voltaic battery; and by varying the resistance it was possible 
to raise it gradually from a state of darkness to an intense 
white heat. Raising it to a white heat in the first instance, 
the rock-salt train was placed in the path of its rays, and a 
brilliant spectrum was obtained. A thermo-pile was then 
moved into the region of obscure rays beyond the red of the 
spectrum. Altering nothing but the strength of the current, 
the spiral was reduced to darkness, and lowered in tem- 
peratuie till the deflection of the galvanometer fell to 1° 
Our question is, ‘ What becomes of the waves which produce 
this deflection when new ones are introduced by augmenting 
the temperature of the spiral ? ’ 

Causing the spiral to pass from this state of darkness through 
various degrees of incandescence, the following deflections were 
obtained:— 


Table I. 


Appearance 
of Spiral 


Deflection by 
obscure rays 

Appearance 
of Spiral 

Deflection by 
obscure rays 

Dark 


O 

. 1 

Full red . 

O 

. 27 

Dark 


. . 6 

Bright red 

. 44*4 

Faint red 


. 10-4 . 

Nearly white . 

. 543 

Dull red . 
Red 


. 12-5 

. 18 

Full white . ' . 

. 60 


The deflection of 60° here obtained is equivalent to 122 of 
the first degrees of the galvanometer. Hence the intensity of 

* See Sections 13 and 14 of Memoir VI. 


17 





258 


LUMINOUS AND OBSCURE RADIATION. 


tlie obscure rays in the case of the full white heat is 122 times 
that of the rays of the same refrangibility emitted by the dark 
spiral used at the commencement. Or, as the intensity is pro¬ 
portional to the square of the amplitude, this, in the case of 
the last deflection, was eleven times that of the waves which 
produced the first. The wave-length, of course, remained the 
same throughout. 

The experimental answer, therefore, to the question above 
proposed is, that the amplitude of the old waves is augmented 
by the same accession of temperature that gives birth to the 
new ones. The case of the obscure rays is, in fact, that of the 
luminous ones (of the red of the spectrum, for example), which 
glow with augmented intensity as the temperature of the 
radiant source of heat is heightened. 

§ 4. 

Persistence and Strengthening of Rays illustrated by means of 
a Ray-filter of Iodine and Bisulphide of Carbon. 

In my last memoir * the wonderful transparency of the 
element iodine to the ultra-red undulations was demonstrated. 
It was there shown that a quantity of iodine sufficient to quench 
the light of our most brilliant flames transmitted 99 per cent, 
of the radiation from a flame of hydrogen. 

Fifty experiments on the radiant heat of a hydrogen flame, 
recently executed, make the transmission of its rays, through a 
quantity of iodine which is perfectly opaque to light, 

100 per cent. 

To the radiation from a hydrogen flame the dissolved iodine is 
therefore, according to these experiments, perfectly transparent 

It is also sensibly transparent to the radiation from solid 
bodies heated "under incandescence. 

It is also sensibly transparent to all the obscure heat-rays 
emitted by luminous bodies. 

To the mixed radiation which issues from solid bodies at a 
very high temperature, the pure bisulphide of carbon is eminently 


* Section 22, Memoir VI. 


LUMINOUS AND OBSCURE RADIATION. 259 

transparent. Hence, as the bisulphide of carbon interferes 
but slightly with the obscure rays issuing from a highly lumi- * 
nous source, and as the dissolved iodine seems not at all to 
interfere with them, we have in a combination of both sub¬ 
stances a means of almost entirely detaching the purely thermal 
rays from the luminous ones. 

If vibrations of a long period, established when the radiating 
body is at a low temperature, maintain themselves, as just in¬ 
dicated, side by side with the new periods which augmented 
temperature introduces, it would follow that a body once per¬ 
vious to the radiation from any source must always remain 
pervious to it. We cannot so alter the character of the radia¬ 
tion that a body once in any measure transparent to it shall 
become quite opaque to it. We may, by augmenting the 
temperature, diminish the percentage of the total radiation 
transmitted by the body; but inasmuch as. the old vibrations 
have their amplitudes enlarged by the very accession of tem¬ 
perature which produces the new ones, the total quantity of 
heat of any given refrangibility transmitted by the body must 
increase with increase of temperature. 

This conclusion is thus experimentally illustrated. A cell 
with parallel sides of polished rock-salt was filled with the solu¬ 
tion of iodine, and placed in front of the camera within which 
was the platinum spiral. Behind the rock-salt cell was placed 
a thermo-electric pile, to receive such rays as had passed 
through the solution. The rock-salt lens was in the camera 
in front, but a small sheaf only of the parallel beam emergent 
from the lamp was employed. Commencing at a very low dark 
heat, the temperature was gradually augmented to full incan¬ 
descence with the following results : — 



Table II. 


Appearance of Spiral 

Deflection 

Appearance of Spiral 

Deflection 

Dark.... 

O 

. 1 

Full red 

O 

. 45 

Dark but hotter. 

. 3 

Bright red. 

. 53 

Dark but still hotter . 

. 5 

Very bright red . 

. 63 

Dark but still hotter . 

. 10 

Nearly white 

. 69 

Feeble red. 

. 19 

White 

. . 75 

Dull red . 

Bed .... 

. 25 
. 35 

Intense white 

, . 80 








260 


LUMINOUS AND OBSCURE' RADIATION. 


To the luminous rays from the intensely white spiral the 
' solution was perfectly opaque; hut though by the introduction 
of such rays the transmission, as expressed in parts of the total 
radiation , was diminished, the quantity absolutely transmitted 
was enormously increased. The value of the last deflection is 
440 times that of the first; by raising therefore the platinum 
spiral from darkness to whiteness, we augment the intensity of 
the obscure rays which it emits in the ratio of 1 : 440. 

A rock-salt cell filled with the transparent bisulphide of carbon 
was placed in front of the camera which contained the platinum 
spiral raised to a dazzling white heat. The transparent liquid 
was then drawn off and its place supplied by the solution of 
iodine. The deflections observed in the respective cases are 
as follows:— 

Radiation from White-hot Platinum. 

Through Transparent CS* Through Opaque Solution 

O o 

73-9 73 

73-8 72-9 

All the luminous rays passed through the transparent bisulphide; 
none of them passed through the solution of iodine. Still we 
see what a small difference is produced by their withdrawal. 
The actual proportion of luminous to obscure, as calculated from 
the above observations, may be thus expressed:— 

Dividing the radiation from a platinum wire raised to a dazzling 
whiteness by an electric current into twenty-four equal parts , one 
of these parts is luminous and twenty-three obscure. 

A bright gas flame was substituted for the platinum spiral, 
the top and bottom of the flame were shut off, and its most 
brilliant portion chosen as the source of rays. The result of 
forty experiments with this source may be thus expressed:— 
Dividing the radiation from the most brilliant portion of a 
flame of coal-gas into twenty-five equal parts, one of those parts is 
luminous and twenty-four obscure. 

I next examined the ratio of obscure to luminous rays in the 
electric light. A battery of fifty cells was employed, and the 
rock-salt lens was used to render the rays from the coal points 
parallel. To prevent the deflection from reaching an incon¬ 
venient magnitude, the parallel rays were caused to issue from 
a circular aperture 0T of an inch in diameter, and were sent 



LUMINOUS AND OBSCURE RADIATION. 


261 


alternately through the transparent bisulphide and through the 
opaque solution. It is not easy to obtain perfect steadiness on 
the part of the electric-light; but three experiments carefully 
executed gave the following deflections :— 

Radiation from Electric-light.—Experiment No. I. 

Through Transparent CS a Through Opaque Solution 

72° 70 o 

Experiment No. II. 

76-5° 750 


Experiment No. III. 

775° 


76-5° 


Calculating from these measurements the proportion of lumi¬ 
nous to obscure heat, the result may be thus expressed:_ 

Dividing the radiation from the electric-light, generated by a 
Grove's battery of fifty cells, into ten equal parts, one of those 
parts is luminous and nine obscure . 

The results may be thus presented in a tabular form:— 

Table III. — Radiation through dissolved Iodine. 


Source of heat 

Absorption per 100 

Transmission 

Dark spiral . 

0 

100 

Lampblack at 212° Fahr 

. 0 

100 

Red-hot spiral 

. 0 

100 

Hydrogen flame 

. 0 

100 

Oil flame 

. 3 

97 

Gas flame 

. 4 

96 

White-hot spiral . 

. 4-6 

954 

Electric-light. 

♦ 

. 10 

90 


Repeated experiments may slightly alter these results, but 
they are extremely near the truth. 


§ 5 . 


Combustion by Invisible Rays. 


Having thus in the solution of iodine found a means of 
almost perfectly detaching the obscure from the luminous heat- 
rays of any source, we are able to operate at will upon the 
former. Here are some illustrations :—The rock-salt lens was 





262 


LUMINOUS AND OBSCURE RADIATION. 


so placed in the camera that the coal points themselves and 
their image beyond the lens were equally distant from the 
latter. A battery of forty cells being employed, the track of 
the cone of rays emergent from the lamp was plainly seen in 
the air, their point of convergence being therefore easily fixed. 
The cell containing the ojmque solution was now placed in front 
of the lamp. The luminous cone was thereby entirely cut off, 
but the intolerable temperature of the focus, when the hand 
was placed.there, showed that the calorific rays were still trans¬ 
mitted. Thin plates of tin and zinc were placed successively 
in the dark focus and speedily fused; matches were ignited, 
gun-cotton exploded, and brown paper set on fire. Employing 
the iodine solution and a battery of sixty of Grove’s cells, all 
these results were readily obtained with the ordinary glass 
lenses attached to Duboscq’s electric lamp. They cannot, I 
think, fail to give pleasure to those who repeat the experiments. 
It is extremely interesting to observe in the middle of the air 
of a perfectly dark room a piece of black paper suddenly pierced 
by the invisible rays, and the burning ring expanding on all 
sides from the centre of ignition. 

On the 15th of this month I made a few experiments on 
solar light. The heavens were not free from clouds, nor the 
London atmosphere from smoke, and at best only a portion of 
the action which a clear day would have given, was obtained. 
I happened to possess a hollow lens, which I filled with the con¬ 
centrated solution of iodine. Placed in the path of the solar 
rays, a faint red ring was imprinted on a sheet of white paper 
held behind the lens, the ring contracting to a faint red spot 
when the focus of the lens was reached. It was immediately 
found that this ring was produced by the liglit which had pene¬ 
trated the thin rim of the liquid lens. Pasting a zone of black 
paper round the rim, the ring was entirely cut off and no visible 
trace of solar light crossed the lens. At the focus, whatever 
light passed would be intensified nine-hundredfold; still even 
here no light was visible. 

Not so, however, with the sun’s obscure rays; the focus was 
burning hot. A piece of black paper placed there was instantly 
pierced and set on fire; and by shifting the paper, aperture 
after aperture was formed in quick succession. Gunpowder 
was also exploded. In fact we had in the focus of the sun’s 


LUMINOUS AND OBSCURE RADIATION. 


263 


dark rays heat decidedly more powerful than that of the 
electric-light similarly condensed, and all the effects obtained 
with the latter could be obtained in an increased decree with 

o 

the former. 

A plano-convex lens of glass, larger than the opaque lens 
just referred to, was introduced into the path of the sun’s rays. 
The focus on white paper was of dazzling brilliancy; and in this 
focus the results already described were obtained. A cell con¬ 
taining a solution of alum was then introduced in front of the 
focus. Tlie intensity of tlie light at the focus was not sensibly 
changed; still these almost intolerable visual rays, aided as they 
were by a considerable quantity of invisible rays which had 
passed through the alum, were incompetent to produce effects 
which were obtained with ease in the perfectly dark focus of the 
opaque lens. 

To show that this reduction of power was not due to the 
withdrawal of heat by reflection from the sides of the glass cell, 
I put in its place a rock-salt cell filled with the opaque solution. 
Behind this cell the rays manifested the power which they 
exhibited in the focus of the opaque lens. 

§ 6 . 

Melloni’s Method of determining the Ratio of Visible to Invisible 
Rays.—Diathermancy of Alum and of the Humours of the Eye. 

Melloni’s experiments led him to conclude that rock-salt 
transmits pbscure and luminous rays equally well, and that a 
solution of alum of moderate thickness entirely intercepts the 
invisible rays, while it allows all the luminous ones to pass. 
Hence the difference between the transmissions of rock-salt and 
alum ought to give the obscure radiation. In this way Melloni 
found that 10 per cent, only of the radiation, from an oil flame 
consists of luminous rays. The method above employed proves 
that the proportion of luminous heat to obscure, in the case of 
an oil flame, is probably not more than one-third of what 
Melloni made it. 

In fact this distinguished man clearly saw the possible inac¬ 
curacy of the conclusion that none but luminous rays are 
transmitted by alum; and the following experiments justify the 
clauses of limitation which he attached to his conclusion:— 


264 


LUMINOUS AND OBSCURE RADIATION. 


The solution of iodine was placed in front of the electric lamp, 
the luminous rays being thereby intercepted. Behind the 
rock-salt cell containing the opaque solution was placed a glass 
cell, empty in the first instance. The deflection produced by 
the obscure rays which passed through both produced a deflec¬ 
tion of 

80°. 

The glass cell was now filled with a concentrated solution of 
alum; the deflection produced by the obscure rays passing 
through both solutions was 

50°. 

Calculating from the values of these deflections, it was found 
that of the obscure heat emergent from the solution of iodine , and 
from the side of the glass cell , 20 per cent, was transmitted by the 
alum . 

A point of very considerable importance forces itself upon our 
attention here—namely, the vast practipal difference which may 
exist between the two phrases, ‘ obscure rays,’ and ‘ rays from 
an obscure source of heat.’ Many writers seem to regard these 
phrases as equivalent to each other, and are thus led into grave 
errors. A stratum of alum solution Jg-th of an inch in thick¬ 
ness is, according to Melloni, entirely opaque to the radiation 
from all bodies heated under incandescence. In the foregoing 
experiments the layer of alum solution traversed by the obscure 
rays of our luminous source of heat was thirty times the thick¬ 
ness of the layer which Melloni found sufficient to quench all 
rays emanating from obscure sources of heat. 

There cannot be a doubt that the invisible rays which have 
shown themselves competent to traverse such a .thickness of 
the most powerful adiathermic liquid yet discovered are also 
able to pass through the humours of the eye. The very careful 
and interesting experiments of M. Janssen,* prove that the 
humours of the eye absorb an amount of radiant heat exactly 
equal to that absorbed by a layer of water of the same thick¬ 
ness, and in our solution the power of alum is added to that of 
water. Direct experiments on the vitreous humour of an ox 
lead me to conclude that one-fifth of the obscure rays emitted 
by an intense electric-light reaches the retina; and inasmuch 

* Annates de Chimie et de Physique, torn, lx, p, 71, 


LUMINOUS AND OBSCURE RADIATION. . 


265 


as m every ten equal parts of the radiation from the carbon 
points nine consist of obscure rays, it follows that nearly two- 
tlnrds of the whole radiant energy which actually reaches the 
retina is incompetent to excite vision. With a white-hot plati¬ 
num spiral as source of heat, the mean of four good experi¬ 
ments gave a transmission of 11*7 per cent, of the obscure 
heat of the spiral through a layer of distilled water 1*2 inch in 
thickness. . A larger proportion no doubt reaches the retina* 

Converging the beam from the electric lamp by a glass lens, 
I placed the opaque solution of iodine before my open eye, and 
brought the eye into the focus of obscure rays; the heat was 
immediately unbearable. But the unpleasant effect seemed to 
be mainly due to the. action of the obscure rays upon the eye¬ 
lids and other opaque parts round the eye. Through an aperture 
in a card, somewhat larger than the pupil, the concentrated 
calorific beam was subsequently permitted to enter the eye. 
The sense of heat entirely disappeared. Not only were the rays 
received by the retina incompetent to excite vision, but the 
optic nerve seemed unconscious of their existence even as heat. 

On a tolerably clear night a candle flame can be readily seen 
at the distance of a mile. The intensity of the electric-light 
used by me is 650 times that of a good composite candle, and 
as the non-luminous radiation from the coal points which 
reaches the retina is equal in energy to twice the luminous, it 
follows that at a common distance of a foot, the energy of the 
invisible rays of the electric-light which reach the optic nerve, 
but are incompetent to provoke vision, is 1,300 times that of 
the light of a candle. But the intensity of the candle’s light 
at the distance of a mile is less than the twenty-millionth of its 
intensity at the distance of a foot, hence the energy which ren¬ 
ders the candle perfectly visible a mile off would have to be 
multiplied by 1,300 x 20,000,000, or by twenty-six thousand 
millions, to bring it up to the energy sent to a retina placed at 
a foot distance from the electric-light, but which, notwith¬ 
standing its enormous relative magnitude, is utterly incompe¬ 
tent to excite vision. Nothing, I think, could more forcibly 
illustrate the special relationship which subsists between the 
optic nerve and the oscillating periods of luminous bodies. The 
nerve may be compared to a musical string, which responds 

* M. Franz has shown that a portion of the sun’s obscure rays reach the retina. 


266 


LUMINOUS AND OBSCUEE RADIATION. 


to periods with, which it is in accordance, while it refuses to 
he excited by others of vastly greater energy which are not 
in unison with its own. 

By means of the opaque solution of iodine, I have already 
shown that the quantity of luminous heat emitted by a bright 
red platinum spiral is immeasurably small.* Here are some 
determinations since made with the same source of heat and a 
solution of iodine in iodide of ethyl, the strength and thickness 
of the solution being such as entirely to intercept the luminous 
rays:— 

Radiation from Red-liot Platinum Spiral. 

Through Transparent Liquid Through Opaque Solution 

o 

43 7 437 

437 437 

These experiments were made with exceeding care, and all 
the conditions were favourable to the detection of the slightest 
difference in the amount of heat reaching the galvanometer; 
still the quantity of heat transmitted by the opaque solution 
was found to be the sa me as that transmitted by the transparent 
one. In other words, the luminous radiation intercepted by 
the former, though competent to excite vividly the sense of 
vision, was, when expressed in terms of actual energy, absolutely 
incapable of measurement. 

And here we have the solution of various difficulties which 
from time to time have perplexed experimenters. When we see 
a vivid light incompetent to affect our most delicate thermo- 
scopic apparatus, the idea naturally presents itself that light and 
heat must be totally different things. The pure light emerging 
from a combination of water and green glass, even when ren¬ 
dered intense by concentration, has, according to Melloni, no 
sensible heating power.t The light of the moon is also a case 
in point. Concentrated by a polyzonal lens more than a yard 
in diameter upon the face of his pile, it required all Melloni’s 
acuteness to nurse the calorific action up to a measurable quan¬ 
tity. Such experiments, however, demonstrate, not that the 
two agents are dissimilar, but that the sense of vision can be 
excited by an amount of power almost infinitely small. 

* Section^, Memoir VI. 
f Taylor s Scie?itijic Memoirs, vol. i. p. 392, 


LUMINOUS AND OBSCURE RADIATION. 


267 


Here also we are able to offer a remark as to tbe applicability 
of radiant heat to fog-signalling.* The proposition, in the 
abstract, is a philosophical one ; for were onr fogs of a physical 
character similar to that of the iodine held in solution by the 
bisulphide of carbon, or to that of iodine or bromine vapour, it 
would be possible to transmit through them powerful beams of 
radiant heat, even after the entire stoppage of the light from 
our signal lamps. But our fogs are not of this character. 
They are unfortunately so constituted as to act very destruc¬ 
tively upon the purely calorific rays; and this fact, taken in 
conjunction with the marvellous sensitiveness of the eye, leads 
to the conclusion that, long before the light of our signals ceases 
to be visible, their radiant heat has lost the power of affecting, 
in any sensible degree, the most delicate thermoscopic apparatus 
that we could apply to their detection. 


Royal Institution, October 1864. 


* Which had been proposed a short time prior to the writing of this paper. 



‘ 


















« 






. 




















































VIII. 


ON CALORESCENCE, OR THE TRANSMUTATION 


HEAT-RAYS. 


ANALYSIS OF MEMOIR VIII. 


The separate memoirs of tliis collection are seen to be so many links in a con¬ 
nected chain of investigation, each new inquiry arising out of some observation 
or suggestion made in those preceding it. 

Thus the diathermancy of the elementary gases led to the discovery of the 
diathermancy of elementary liquids, embracing solutions of sulphur and iodine. 
This again led to the experiments made in 1862 on the formation of invisible 
foci by the filtered beam of the electric-light. The possession of rock-salt 
lenses in 1884 enabled me to ignite combustible substances at the dark 
focus; and having obtained this mastery over the subject, it was found possible, 
by adopting the precise arrangement employed in March 1862, to produce 
combustion. 

The present memoir is a further step in this direction. Reference is first 
made to the discovery of the sun’s invisible rays by Sir William Herschel. 
The results of Professor Muller are also referred to, and experiments are described 
in which the distribution of heat in the spectrum of the electric-light is strictly 
determined by measurement, and represented graphically. • 

It is thus proved by experiments involving prismatic analysis that the 
thermal energy of the invisible radiation of a very powerful electric-light is 
eight times that of the visible. 

The same result is deduced from experiments with the iodine-Jilter. 

Various efforts made to intensify the dark foci of the electric-light; the 
introduction and improvement of small concave mirrors with a view to con¬ 
centration, and the greatly augmented effects of combustion at the dark foci 
thus intensified, are fully described and illustrated. 

In Memoir VI. a vibrating molecule of aqueous vapour is compared with a 
vibrating particle of the luminiferous aether, and it is contended that, as regards 
the change of period produced by rendering refractory bodies incandescent, the 
principle is the same. This conclusion is confirmed. For it is proved that a 
sheet of platinum, which when plunged into a flame of hydrogen becomes 
white-hot, is also heated to whiteness at the perfectly dark focus of the filtered 
electric beam. 

From the dark rays thus transmuted by the platinum , a spectrum may be 
obtained embracing all the visual vaysfrom red to blue . 

A perfectly invisible image of the carbon points is shown to be formed by the 
heat-rays, the image being changed to a bright incandescent one when the in¬ 
visible rays fall on platinised platinum. 

It is shown that the eye can be placed, without inconvenience, at a dark focus 
sufficiently intense to heat platinum to redness. 

This change of vibration from slow to quick, and from unvisual to visual 
periods, is called calorescence. 


ANALYSIS OF MEMOIR VIII. 


271 


The precautions needed in dealing with the inflammable bisulphide of carbon 
aie fully dwelt upon, and various methods of handling the dark rays with 
safety and certainty are described. 

Experiments on the calorescence of the sun’s obscure rays and those of the 
lime-light are also described. 

Experiments on the relation of colour to combustion by the dark rays are 
also recorded. 

Calorescence with ether filters than the iodine one is also shown to be 

possible. The memoir winds up by some remarks on the defects of the black- 
bulb thermometer. 

































» 
































Yin. 


ON CALORESCENCE, OR THE TRANSMUTATION 

OF HEAT RAYS.* 


Forsitan et rosed sol alt& lampade lucens 
Possideat multum csecis fervoribus ignem 
Circum se, nullo qui sit fulgore notatus, 

JEstiferum ut tantum radiorum exaugeat ictum. 

__ Lucket. y. 610.f 


* § 1 . 

General Statement of the Nature of this Inquiry. 

In the year 1800, and in the same volume of the £ Philosophical 
Transactions * that contains Volta’s celebrated letter to Sir 
Joseph Banks on the Electricity of Contact,! Sir William 
Herschel published his discovery of the invisible rays of the 
sun. Causing thermometers to pass through the various 
colours of the solar spectrum, he determined their heating- 

tD 

power, and found that this power, so far from ending at the 
red extremity of the spectrum, rose to a maximum at some dis¬ 
tance beyond the red. The experiment proved that, besides its 
luminous rays, the sun emitted others of low refrangibility, 
possessing great calorific power, but incompetent to excite 
vision. 

Drawing a datum-line to represent the length of the spec¬ 
trum, and erecting at various points of this line perpendiculars 
to represent the calorific intensity existing at those points, on 
uniting the ends of the perpendiculars Sir William Herschel 

* Received October 20th, and read before the Royal Society, November 23, 1865; 
Philosophical Transactions for 1866, p. 1 ; Philosophical Magazine for May and June 
1866. The phrase ‘ transmutation of rays ’ is, 1 believe due to Professor Challis. 

t I am indebted to my excellent friend Sir Edmund Head for this extract. 

J Vol. lxx. 


18 




274 


OX CALORESCENCE, OR THE 


obtained the subjoined curve (fig. 19), which shows the distribu¬ 
tion of heat in the solar spectrum, according to his observations. 
The space ABD represents the invisible, and BDE the visible 
radiation of the sun. 

Fig. 19. 


13 



A £ E 

SPECTRUM OF SUN (HERSCHEL) REDUCED. 


With the more perfect apparatus subsequently devised, Pro¬ 
fessor Muller of Freiburg examined the distribution of heat 
in the spectrum,* and the results of his observations are rendered 
graphically in fig. 20. Here the area ABCD represents the 
invisible, while C D E represents the visible radiation. 


B 


Fig. 20. 



SPECTRUM OF SUN (mUEEEr). 


With regard to terrestrial sources of heat, it may be stated 
that all such sources hitherto examined emit those obscure rays. 
Melloni found that 90 per cent, of the emission from an oil 
flame, 98 per cent, of the emission from incandescent platinum, 
and 99 per cent, of the emission from an alcohol flame consists 
of obscure rays.f The visible radiation from a hydrogen flame 
is, according to my own experiments, too small to admit of 
measurement. With regard to solid bodies, it may be stated 
generally that, when they are raised from a state of obscurity 
to vivid incandescence, the invisible rays emitted in the first 

* Philosophical Magazine, S. 4, vol. xvii. p. 242. 
f La Thernxochrose, p. 304, 







transmutation of heat rays. 275 

instance continue to be emitted with augmented power when 
the body glows. For example, with a current of feeble power 
the carbons of the electric lamp may be warmed and caused to 
emit invisible rays. But the intensity of these same rays may 
be augmented a thousandfold by raising the carbons to the 
temperature necessary for the electric-light. Here, in fact, the 
luminous and non-luminous emission augment together, the 
maximum of brightness of the visible rays occurring simul¬ 
taneously with the maximum calorific power of the invisible 
ones.* 

At fiequent intervals during the past ten or twelve years I 
have had occasion to experiment on the invisible rays of the 
electric-light, and the discovery of the iodine-filter enables me 
now to make them the subject of special investigation. I en¬ 
deavour, in the first place, to compare the luminous with the 
non-luminous radiation of the electric-light, and to determine 
their relative energy; then a method is pointed out of detaching 
the luminous from the non-luminous rays, and of concentrating 
the latter in intense invisible foci. Various experiments 
illustrative of the calorific power of the invisible rays, and of 
their transmutation into visible ones are afterwards described. 

§ 2 . 

Source of Rays.—Employment of Rod-salt Train. 

Through the kindness of my friend Mr. Gassiot, a very beauti¬ 
ful linear thermo-electric pile, constructed by Ruhmkorff, has 
remained in my possession for several years, and been frequently 
employed in my researches. It consists of a double metallic 
screen, with a rectangular aperture in the centre, a single row 
of thermo-electric elements 1-2 inch in length being fixed to 
the screen behind the aperture. Connected with the latter are 
two moveable side pieces, which can be caused to approach or 
recede so as to vary the width of the exposed face of the pile 
from 2 fero to T ^th of an inch. The instrument is mounted on 
a slider, which, by turning a handle, is gradually moved along 
a massive metal stand. A spectrum of a width equal to the 
length of the thermo-electric pile being cast at the proper 

* On this point see the Bede Lecture for 1865, p. 33 (Longmans). Reprinted iu 
Fragments of Science , 1871 (Longmans). 


276 


ON CALORESCENCE, OR THE 


elevation on the screen, by turning 1 the handle of the slider the 
vertical face of the pile can be caused to traverse the colours, 
and also the spaces right and left of them. 

To produce a steady spectrum of the electric-light, I em¬ 
ployed the regulator devised by M. Foucault and constructed by 
Duboscq, the constancy of which is admirable. A complete 
rock-salt train of high transparency, constructed for me by 
Mr. Becker, was arranged in the following manner:—In the 
camera was placed a rock-salt lens, which reduced to paral¬ 
lelism the divergent rays proceeding from the carbon points. 
The parallel beam was permitted to pass through a narrow 
vertical slit. In front of this was another rock-salt lens, which 
produced a sharply-defined image* of the slit at a distance 
beyond it equal to that at which the spectrum was to be 
formed. Immediately behind this lens was placed a pure 
rock-salt prism with its axis vertical—sometimes a pair of 
prisms. The beam was thus decomposed, a brilliant horizontal 
spectrum being cast upon the screen which bore the thermo¬ 
electric pile. By turning the handle already referred to, the 
face of the pile could be caused to traverse the spectrum, an 
extremely narrow band of light or radiant heat falling upon 
it at each point of its march.| The pile was connected with 
an exceedingly sensitive galvanometer, by which the heating- 
power of every part of the spectrum, visible and invisible, was 
determined. 


§ 3. 

Methods of Experiments and Tabulated Results. 


Two modes of moving the instrument were practised. In 
the first the face of the pile was brought up to the violet end 
of the spectrum, where the heat was insensible, and then 
moved through the colours to the red, then past the red up to 
the position of maximum heat, and afterwards beyond this 
position until the heat of the invisible spectrum gradually 
faded away. The following table contains a series of measure¬ 
ments executed in this manner. The motion of the pile is 
measured by turns of its handle, every turn corresponding to the 
shifting of the face of the instrument through a space of one 

* The width of the image was about 0*1 of an inch, 
f The width of the linear pile was 0‘03 of an inch. 


TRANSMUTATION OF HEAT RAYS. 277 

millimetre, or ^-th of an inch. At the beginning, where the 
increment of heat was slow and gradual, the readings were 
taken at every two turns of the handle; on quitting the red, 
where the heat suddenly increases, the intervals were only half 
a turn, while near the maximum, where the changes were most 
sudden, the intervals were reduced to a quarter of a turn, which 
corresponded to a translation of the pile through -j^th of an 
inch. Intervals of one and of two turns were afterwards 
resumed until the heating-power ceased to be distinct. At every 
halting-place the deflection of the needle was noted, and its value 
ascertained from the table of calibration. 

It was found convenient to call the maximum effect in each 
series of experiments 100. The first column of figures in the 
table gives the values of the deflections, expressed in terms of 
the lowest degree of the galvanometer; the second column, 
obtained by multiplying the first by the constant factor 1*37, 
expresses the heat of all the parts of the snectrum with refer¬ 
ence to the maximum of 100. 


Table I .—Distribution of Heat in 

Spectrum of Electric Light . 

Movement of Pile 

Value of 
deflection 

Calorific intensity, 
in lOOths of the 
maximum 

Before starting (pile in the blue). 

0 

0 

Two turns forward (green entered) 

1-5 

2 

u • • • • 

3-5 

4-8 

n • • • • 

0-5 

75 

,, (red entered) 

15-5 

21 

„ (extreme red) . 

32-6 

446 

Half turn forward 

44 

60 

n • • • • 

54 

74 

5) • • • • 

62 

85 

>J • • • • 

70 

95-8 

99 • • • • 

72-5 

99 

Quarter turn forward, maximum '■. 

73 

100 

99 • • • 

70-8 

97 

Half turn forward 

57 

78 

,, • • • • 

45-5 

62 

• • • • 

32-6 

44*5 

99 • • • • 

26 

35-6 

Two turns forward 

10-5 

144 

99 • • . • 

6-5 

9 

99 • • • • 

5 

68 

99 .... 

3-5 

5 

99 .... 

25 

34 

.... 

1*7 

2-3 

09 .... 

• 

1-3 

1-8 





278 


ON CALORESCENCE, OR THE 


Here, as before stated, we begin in the blue, and pass first 
through the whole visible spectrum. Quitting this at the 
place marked c extreme red,’ we enter the invisible calorific 
spectrum and reach the position of maximum heat, from 
which, onwards, the thermal power falls till it practically dis¬ 
appears. 

In other observations the pile was first brought up to the 
position of maximum heat, and moved thence to the extremity 
of the spectrum in one direction. It was then brought back to 
the maximum, and moved to the extremity in the other direc¬ 
tion. There was generally a small difference between the two 
maxima, arising, no doubt, from some slight alteration of the 
electric-light during the period which intervened between the 
two observations. The following table contains the record 
of a series of such measurements. As in the last case, the 
motion of the pile is measured by turns of the handle, and the 
values of the deflections are given with reference to a maximum 
of 100. 


Table II. —Distribution of Heat in Spectrum of Electric Light . 


Movement of Pile 

Maximum. 

One turn towards visible spectrum 


Calorific intensity, 
in lOOths of the 
maximum 
100 

94*4 


a 

99 

99 

99 

*9 


99 

99 

99 

99 

99 

99 


(extreme red) 


Two turns in the same direction (green entered) 

• • 

• • 

(pile in blue) 


M 

»» 

ft 


it 

it 

it 


655 

426 

283 

20 

14-8 

11-1 

7-4 

4*6 

2 

0-9 


Tile brought back to maximum. 


Maximum . 

One turn from visible spectrum . 


99 


99 

99 


99 

99 


99 

99 


99 

Two turns . 

• 

• • • 

a • 

• 

• • • 

• 

« 

• • • 


100 

67*1 

41 

23 

13 

9*4 

5 

3-4 

0 






TRANSMUTATION OP HEAT RAYS. 


279 


§ 4 . 

Graphic Representation of Results.—Curve of the Electric Spectrum. 

—Deviations from Solar Spectrum. 

More than a dozen series of such measurements were exe¬ 
cuted, and afterwards plotted as ordinates from a datum-line 
representing the length of the spectrum. Uniting the ends of 
these ordinates a number of curves were obtained, each of 
which represented the distribution of heat in the spectrum, as 
shown by the corresponding series of observations. On super¬ 
posing, by means of tracing-paper, the different curves, a very 
close agreement was found to exist between them. The annexed 
diagram (fig. 21), which is the mean of several, expresses, with a 
close approximation to accuracy, the distribution of heat in the 
spectrum of the electric-light from fifty cells of Grove. The 
space A B 0 D represents the invisible, while C D E represents 
the visible radiation. We observe the gradual augmentation of 
thermal power, from the blue end of the spectrum to the red. 
But in the region of dark rays beyond the red the curve shoots 
suddenly upwards in a steep and massive peak, which quite 
dwarfs by its magnitude the portion of the diagram repre¬ 
senting the visible radiation.* 

The sun’s rays before reaching the earth have to pass through 

* How are we to picture the vibrating atoms which produce the different wave¬ 
lengths of the spectrum? Does the infinity of the latter, between the extreme ends 
of the spectrum, answer to an infinity of atoms each oscillating at a single rate ? 
or are we not to figure the atoms as virtually capable of oscillating at different rates 
at the same time ? When a sound and its octave are propagated through the same 
mass of air, the resultant motion of the air is the algebraic sum of the two separate 
motions impressed upon it. The ear decomposes this motion into its two components 
(Helmholtz, Ton-Empfindungcn, p. 54); still we cannot here figure certain particles of 
the air occupied in the propagation of the one sound, and certain other particles in the 
propagation of the other. May not what is true of the air be true of the sether ? and 
may not, further, a single atom, controlled and jostled as it is in solid bodies by its 
neighbours, be able to impress upon the sether a motion equivalent to the sum of 
the motions of several atoms each oscillating at one rate ? 

It is perhaps worthy of remark, that there appears to be a definite rate of vibration 
for all solid bodies having the same temperature, at which the vis viva of their atoms 
is a maximum. If, instead of the electric-light, we examine the lime-light, or a 
platinum wire raised to incandescence by an electric current, we find the apex of the 
curve of distribution (B, fig. 21) corresponding throughout to very nearly, if not exactly, 
the same refrangibility. There seems, therefore, to exist one special rate at which the 
atoms of heated solids oscillate with greater energy than at any other rate—a non¬ 
visual period, which lies about as far from the extreme red of the spectrum on the 
invisible side as the commencement of the green on the visible one. 


280 


OX CALORESCENCE, OR THE 


our atmosphere, where they encounter the atmospheric aqueous 
vapour, which exercises a powerful absorption on the invisible 



calorific rays. From this, apart from other considerations, it 
would follow that the ratio of the invisible to the visible radia- 








TRANSMUTATION OF HEAT RAYS. 


281 


tion in the case of the sun must be less than in the case of the 
electric-light. Experiment, we see, justifies this conclusion; 
for whereas fig. 20 shows the invisible radiation of the sun to be 
about twice the visible, fig. 21 shows the invisible radiation of 
the electric-light to be nearly eight times the visible. If we 
cause the beam from the electric lamp to pass through a layer 
of water of suitable thickness, we place its radiation in approxi¬ 
mately the same condition as that of the sun; and on decompo- 
sing the beam after it has been thus sifted, we obtain a dis¬ 
tribution of heat closely resembling that observed in the solar 
spectrum. 

The curve representing the distribution of heat in the electric 
spectrum falls most steeply on that side of the maximum which 
is most distant from the red. On both sides, however, we have 
a continuous falling-off. I have had numerous experiments made 
to ascertain whether there is any interruption of continuity in 
the calorific spectrum; but all the measurements hitherto exe¬ 
cuted with artificial sources reveal a gradual and continuous 
augmentation of heat from the point where it first becomes 
sensible up to the maximum. Sir John Herschel has shown 
that this is not the case with the radiation from the sun when 
analysed by a flint-glass prism. Permitting the solar spectrum 
to fall upon a sheet of blackened paper, over which had been 
spread a wash of alcohol, this eminent philosopher determined 
by its drying-power the heating-power of the spectrum. He 
found that the wet surface dried in a series of spots representing 
thermal maxima separated from each other by spaces of com¬ 
paratively feeble calorific intensity. Ho such maxima and 
minima were observed in the spectrum of the electric-light, nor 
in the spectrum of a platinum wire raised to a white heat by 
a voltaic current. Prisms and lenses of rock-salt, of crown 
glass, and of flint glass were employed in these cases. In 
subsequent experiments the beam intended for analysis was 
caused to pass through layers of water and other liquids of 
various thicknesses. Gases and vapours of various kinds 
were also introduced into the path of the beam. In all cases 
there was a general lowering of the calorific power, but the 
descent of the curve on both sides of the maximum was un¬ 
broken.* 


* At a future day I hope to subject this question to a more severe examination. 


282 


OX CALORESCENCE, OR THE 


§ 5 . * 

Rays from Obscure Sources of Heat contrasted with Obscure Rays 
from Luminous Sources of Heat.—Further Observations on the 

Construction of a Ray-filter. 

Tlie rays from an obscure source of heat cannot compete in 
point of intensity with the obscure rays of a luminous source of 
heat. No body heated under incandescence could emit rays of 
an intensity comparable to those of the maximum region of the 
electric spectrum. If therefore we wish to produce intense 
calorific effects by invisible rays, we must choose those emitted 
by an intensely luminous source of heat. The question then 
arises, how are the invisible calorific rays to be isolated from 
the visible ones ? The interposition of an opaque screen suffices 
to cut off the visible spectrum of the electric-light, and leaves 
us the invisible calorific rays to operate upon at our pleasure. 
Sir William Herschel experimented thus when he sought, by 
concentrating them, to render the invisible rays of the sun 
visible. But to form a spectrum in which the invisible rays 
shall be completely separated from the visible ones, a narrow 
slit or a small aperture is necessary; and this circumstance 
renders the amount of heat separable by prismatic analysis 
Very limited. If we wish to ascertain what the intensely con¬ 
centrated invisible rays can accomplish, we must devise some 
other mode of detaching them from their visible companions. 
We must, in fact, discover a substance which shall filter the 
composite radiation of a luminous source of heat by stopping 
the visible rays and allowing the invisible ones free transmission. 

Could we obtain a black elementary body thoroughly homo¬ 
geneous, and with all its parts in perfect optical contact, experi¬ 
ments already published would lead me to expect that such a 
body would form an effectual filter for the radiation of the sun 
or of the electric-light. While cutting off the visible radiation, 
the black element would, I imagine, allow the invisible to pass. 
Carbon in the state of soot is black, but its parts are not optically 
continuous. In black glass the continuity is far more perfect, 
and hence the result established by Melloni, that black glass 
possesses a considerable power of transmission. Gold in ruby 
glass, or in the state of jelly prepared by Mr. Faraday, I find to 


TRANSMUTATION OF HEAT RAYS. 


283 


* 


be exceedingly transparent to the invisible calorific rays, but it 
is not black enough to quench the visible ones. The densely 
brown liquid bromine is better suited to our purpose; for, in 
thicknesses sufficient to quench the light of our brightest flames, 
this element displays extraordinary diathermancy. Iodine can¬ 
not be applied in the solid condition, but it dissolves freely in 
■various liquids, the solution in some cases being intensely dark. 
Here, however, the action of the element may be masked by that 
of its solvent. Iodine, for example, dissolves freely in alcohol; 
but then alcohol is so destructive of the ultra-red rays that it 
would be entirely unfit for experiments the object of which is to 
retain these rays while quenching the visible ones. The same 
remark applies in a greater or less degree to most other sol¬ 
vents of iodine. 

The deportment of bisulphide of carbon, both as a vapour and 
a liquid, suggests the thought that it would form a most suit¬ 
able solvent. It is extremely diathermic, and there is hardly 
another substance able to hold so large a quantity of iodine in 
solution. Experiments already recorded prove that, of the rays 
emitted by a red-hot platinum spiral, 94*5 per cent, is trans¬ 
mitted by a layer of the liquid (M)2 of an inch in thickness, the 
transmission through layers 0*07 and 0*27 of an inch thick 
being 87*5 and 82*5 respectively.* The following experiment 
with a layer of far greater thickness exhibits the deportment of 
the transparent bisulphide towards the more intense radiation of 
the electric-light. A cylindrical cel], 2 inches in length and 
2*8 inches in diameter, with its ends stopped by plates of per¬ 
fectly transparent rock-salt, was placed empty in front of an 
electric lamp; the radiation from the lamp, after having crossed 
the cell, fell upon a thermo-electric pile, and produced a deflec¬ 
tion of 

73°. 

Leaving the cell undisturbed, the transparent bisulphide of 
carbon was poured into it: the deflection fell to 

72°. 

A repetition of the experiment gave the following results:—* 

* Philosophical Transactions , vol. clir. p. 333; Philosophical Magazine , S. 4, 
rol. xxviii, p. 446, 


284 


ON CALORESCENCE, OR THE 


Deflection 

O 

Through empty cell . . . .74 

Through bisulphide . . . .73 

Taking the values of these deflections from a table of calibra¬ 
tion and calculating the transmission, that through the empty 
cell being 100, we obtain the following results :— 

Transmission 

For the first experiment . . . 94-9 

For the second experiment. . . 94-6 

Mean . . .94-8 

Hence the introduction of the bisulphide lowers the transmis¬ 
sion only from 100 to 94'8.* 

The vehicle which holds the iodine in solution would, if 
perfect for our purpose, be perfectly transparent to the 
total radiation ; and the bisulphide of carbon is shown by the 
foregoing experiment to approach tolerably near perfection. 
We have in it a body capable of transmitting with little loss the 
entire radiation of the electric-light. Our object is now to filter 
this total, by the introduction into the bisulphide of a substance 
competent to quench the visible and transmit the invisible rays. 
Iodine does this with marvellous sharpness. In a short paper 
6 On Luminous and Obscure Radiation,’ published in the c Phi¬ 
losophical Magazine’ for November 1864,t the diathermancy of 
this substance is illustrated by the following table :— 

Table III. — Radiation through dissolved Iodine. 


Source 

Transmission 

Dark spiral of platinum wire 

. 100 

Lampblack at 212° Fahr. . 

. 100 

Red-hot platinum spiral 

. 100 

Hydrogen flame .... 

. 100 

Oil flame ..... 

. 97 

Gas flame ..... 

. 96 

White-hot spiral 

. 954 

Electric-light, battery of 50 cells 

. 90 


These experiments were made in the following way:—A rock- 
salt cell was first filled with the transparent bisulphide, and the 
quantity of heat transmitted by the pure liquid to the pile was 
determined. -The same cell was afterwards filled with the opaque 

* The diminution of the reflexion from the sides of the cell by the introduction of 
the bisulphide is not here taken into account, 
f Being Memoir VII. of this volume. 





TRANSMUTATION OF HEAT RAYS. 


285 


solution, the transmission through which was also determined. 
Calling the transmission through the transparent liquid 100, 
the foregoing table gives the transmission through the opaque. 
The results, it is plain, refer solely to the iodine dissolved in the 
bisulphide,—the transmission 100, for example, indicating, not 
that the solution itself, but that the dissolved iodine is, within 
the limits of observation, perfectly diathermic to the radiation 
from the first four sources of heat. 

The layer of liquid employed in these experiments was not 
sufficiently thick to quench utterly ihe luminous radiation from 
the electric lamp. A cell was therefore constructed whose 
parallel faces were 2*3 inches apart, and which, when filled with 
the solution of iodine, allowed no trace of the most highly 
concentrated luminous beam to pass through it. Five pairs 
of experiments executed with this cell yielded the following 
results :— 

Radiation from Electric Light; battery 40 cells . 

Deflection 


O O 


/Through transparent bisulphide 

. 47 

46 

l Through opaque solution 

. 42-3 

43-5 

/Through transparent bisulphide 

. 44 

43-7 

\ Through opaque solution 

. 41-2 

40 

Through transparent bisulphide 

. 42 

43 


Calling the transmission through the transparent liquid 100, 
and taking the mean of all these determinations, the transmis¬ 
sion through the opaque solution is found by calculation to be * 
86*8. An absorption of 13*2 per cent, is therefore to be set 
down to the iodine. This was the result with a battery of forty 
cells ; subsequent experiments with a battery of fifty cells made 
the transmission 89, and the absorption 11 per cent. 

Considering the transparency of the iodine for heat emitted 
by all sources heated up to incandescence, as exhibited in Table 
III., it may be inferred that the above absorption of 11 per 
cent, represents the calorific intensity of the luminous rays 
alone. By the method of filtering, therefore, we make the 
invisible radiation of the electric-light eight times the visible. 
Computing, by means of a proper scale, the area of the spaces 
A B C D, C D E (fig. 21), the former, which represents the invi¬ 
sible emission, is found to be 7*7 times the latter. Prismatic 



286 


ON CALORESCENCE, OR THE 


analysis, therefore, and the method of filtering yield almost exactly 
the same result. 

§ 6 . 

Invisible Foci of Electric Light.—Efforts to intensify their Heat. 

Hanger of Bisulphide of Carbon, and trial of other substances . 

—Final Precautions. 

In the combination of bisulphide of carbon and iodine we 
find a means of filtering the composite radiation from any 
luminous source. The solvent is practically transparent, while 
the dissolved iodine cuts off every visible ray, its absorptive 
power ceasing with extraordinary suddenness at the extreme red 
of the spectrum. Doubtless the absorption extends a little way 
beyond the red, and with a very great thickness of solution the 
absorption of the ultra-red rays might become very sensible. 
But the solution may be employed in layers which, while com¬ 
petent to intercept every trace of light, allow the invisible calo¬ 
rific rays to pass with scarcely sensible diminution. 

The ray-filter here described was first publicly employed in 
the early part of 1862.* Concentrating by large glass lenses 
the radiation of the electric lamp, I cut off the visible portion of 
the radiation by the solution of iodine, and thus formed invisible 
foci of an intensity at that time unparalleled. In the autumn 
of 1864 similar experiments were executed with rock-salt lenses 
and with mirrors. The paper ‘On Luihinous and Obscure 
Badiation,’ already referred to, contains an account of various 
effects of combustion and fusion which were then obtained with 
the invisible rays of the electric-light and of the sun.f 

* Philosophical Transactions , 1862. p. 67, note. 

f To the experiments there described the following may bo added, as made at the 
time:—A glass, globe 3J inches in diameter, was filled with the opaque solution, and 
placed in front of the electric-light. An intense focus of invisible rays was formed 
immediately beyond the globe. Black paper held in this focus was pierced, a burning 
ring being produced. A second spherical flask, 9 inches in diameter, was filled with 
the solution and employed as a lens. The effects, however, were less powerful than 
those obtained with the smaller flask. 

Two plano-convex lenses of rock-salt, 3 inches in diameter, were placed with their 
plane surfaces opposed, but separated from each other by a brass ring |ths of an inch 
thick. The space between the plates was filled with the solution, an opaque lens being 
thus formed. Paper was fired by this lens. In none of these cases, however, could 
the paper be caused to blaze. Hollow plano-convex lenses filled with the solution 
were not effective, the focal length of those at my disposal being too great. 

Mr. Mayall was so extremely obliging as to transfer his great photographic camera 


TRANSMUTATION OF HEAT RAYS. 287 

From the setting of paper on fire, and the fusion of non¬ 
refractory metals, to the rendering of refractory bodies incandes¬ 
cent, the step was immediate. To avoid waste by conduction, 
it was necessary to employ the metals in plates as thin as pos¬ 
sible. A few preliminary experiments with platinum-foil, which 
resulted in failure, raised the question whether, even with the 
total radiation of the electric-light, it would be possible to obtain 
incandescence without combustion. Abandoning the use of 
lenses altogether, I caused a thin leaf of platinum to approach 
the ignited coal points. It was observed by myself from behind, 
while my assistant stood beside the lamp, and looking through 
a dark glass, observed the distance between the platinum-foil 
and the electric-light. At half an inch from the carbon points 
the metal became red-hot. The problem now was to obtain , at 
a greater distance , a focus which should possess a heating-power 
equal to that of the direct rays at a distance of half an inch. 

In the first attempt the direct rays were utilized as much as 
possible. A piece of platinum-foil was placed an inch distant 
from the carbon points, there receiving the direct radia¬ 
tion. The rays emitted backwards from the points were at the 
same time converged upon the foil by a small mirror, and were 
found more than sufficient to compensate for the diminution of 
intensity due to the withdrawal of the foil to the distance of an 
inch. By the same method incandescence was subsequently 
obtained when the foil was removed two, and even three, inches 
from the carbon points. 

The last-mentioned distance allowed me to introduce between 
the focus and the source of rays a cell containing the solution 
of iodine. The transmitted invisible rays were found of suffi¬ 
cient power to inflame paper, and to raise platinum-foil to 
incandescence. 

These experiments, however, were not unattended with 
danger. The bisulphide of carbon is an extremely inflammable 
substance; and on the 2nd of November, while employing a 
very powerful battery and intensely heated carbon points, the 


from Brighton to London, for the purpose of enabling me to operate with the fine 
glass lens, 20 inches in diameter, which belonged to it: the result was not successful. 
It will, however, be subsequently shown that both the hollow lens and the glass lens 
are effective when, instead of the divergent rays of the electric lamp, we employ the 
parallel rays of the sun. 


288 


ON CALORESCENCE, OR THE 


solution took fire, and instantly enveloped the electric lamp 
and all its appurtenances in flame. The precaution, however, 
had been taken of placing the entire apparatus in a flat vessel 
containing water, into which the flaming mass was summarily 
turned. The bisulphide of carbon being heavier than the 
water, sank to the bottom, so that the flames were speedily 
extinguished. Similar accidents occurred twice subsequently. 

Such occurrences caused me to seek earnestly for a substitute 
for the bisulphide. Pure chloroform, though not so diathermic, 
transmits the obscure rays pretty copiously, and it freely dis¬ 
solves iodine. In layers of the thickness employed, however, 
the solution was not sufficently opaque; and in consequence of 
its absorptive power, comparatively feeble effects only were 
obtained with it. The same remark applies to the iodides of 
methyl and ethyl, to benzol, acetic ether, and other sub¬ 
stances. They all dissolve iodine, but they enfeeble the results 
by their action on the ultra-red rays. 

I had special cells constructed for bromine and chloride of 
sulphur: neither of these substances is inflammable; but they 
are both intensely corrosive, and their action upon the lungs 
and eyes was so irritating as to render their employment im¬ 
practicable. With both of these liquids powerful effects were 
obtained; still their diathermancy, though very high, did not 
come up to that of the dissolved iodine. Bichloride of carbon 
would be invaluable if its solvent power were equal to that of 
the bisulphide. It is not at all inflammable, and its own 
diathermacy appears to excel that of the bisulphide. But in 
reasonable thicknesses the quantity of iodine which it can dis¬ 
solve is not sufficient to render the solution perfectly opaque. 
The solution forms a purple colour of indescribable beauty. 
Though unsuited to strict crucial experiments on dark rays, 
this filter may be employed with good effect in lecture experi¬ 
ments.' 

Thus foiled in my attempts to obtain a solvent equally good 
and less dangerous than the bisulphide of carbon, I sought to 
reduce to a minimum the danger of employing this substance. 
At an earlier period of the investigation a tin camera was 
constructed, within which were placed both the lamp and its 
mirror. Through an aperture in front, 2f inches wide, the 
cone of reflected rays issued, forming a focus outside the 


TRANSMUTATION OF IIEAT RAYS. 


239 


camera. Underneath, this aperture was riveted a stage, on 

which the solution of iodine rested, covering the aperture and 

cutting off all the light. In the first experiments nothing 

intervened between the cell and the carbon points; but the 

peril of thus exposing the bisulphide caused me to make the 

following improvements First, a perfectly transparent plate 

of rock-salt, secured in a proper cap, was employed to close 

the aperture; and by it all direct communication between the 

• 


Fi g- 22. Fig. 23. 



solution and the incandescent carbons was cut off. The camera 
itself, however, became quickly heated by the intense radiation 
falling upon it, and the cell containing the solution was liable 
to be warmed both by the camera and by the luminous heat 
which it absorbed. The aperture above referred to was there¬ 
fore surrounded by an annular space, about 2-J inches wide and 
a quarter of an inch deep, through which cold water was 
caused to circulate. The cell containing the solution was 
moreover surrounded by a jacket, and the current, having com¬ 
pleted its course round the aperture, passed round the solution. 

19 

































































































































































































































290 


ON CALORESCENCE, OR THE 


Tims the apparatus was kept cold. The neck of the cell was 
stopped by a closely-fitting cork; through this passed a piece 
of glass tubing, which, when the cell was placed upon its 
stage, ended at a considerable distance from the focus of the 
mirror. Experiments on combustion might therefore be carried 
on at the focus without fear of igniting the small amount of 
vapour which even under the improved conditions might escape 
from the bisulphide of carbon. 

The arrangement will be at once understood by reference to 
figs. 22 and 23, which show the camera, lamp, and filter both 
from the side and from the front, x y, fig. 22, is the mirror, 
from which the reflected cone of rays passes, first, through 
the rock-salt window and afterwards through the iodine 
filter m n. The rays converge to the focus h, where they form 
an invisible image of the lower carbon point; the image of the 
upper one being thrown below Jc. Both images spring vividly 
forth when a leaf of platinized platinum is exposed at the focus . 
At s s, fig. 22, is shown, in section, the annular space in which 
the cold water circulates. Fig. 23 shows the manner in which 
the water enters this space and passes from it to the jacket 
surrounding the iodine-cell m. 


§ 7 . 


Fig. 24. 


Calorific Effects at Invisible Focus.—Placing of the Eye there . 

With the foregoing arrangement, and a battery of fifty cells, 
the following results were obtained :— 

A piece of silver-leaf, fastened to a 
wire ring and tarnished by exposure to 
the fumes of sulphide of ammonium, 
being held in the dark focus, the film 
flashed out occasionally into vivid red¬ 
ness. 

Copper-leaf tarnished in a similar 

manner, when placed at the focus, was 

\ 

heated to redness. 

A piece of platinized platinum-foil 
was supported in an exhausted receiver, the vessel being so 
placed that the focus fell upon the platinum. The heat of the 
focus was instantly converted into light, a clearlv-defined and 




TRANSMUTATION OF HEAT RAYS. 291 

inverted image of the points being stamped upon the metal. 
Fig. 24 represents the thermograph of the carbons. 

Blackened paper was now substituted for the platinum in the 
exhausted receiver. Placed at the focus of invisible rays, the 
paper was instantly pierced, a cloud of smoke was poured 
through the opening, and fell like a cascade to the bottom of 
the receiver. The paper seemed to burn without incandescence. 
Here also a thermograph of the coal points was stamped out. 
When black paper is placed at the focus, where the thermal 
image is well defined, it is always pierced in two points, answer¬ 
ing to the images of the two carbons. The superior heat of 
the positive carbon is shown by the fact that its image first 
pierces the paper; it burns out a large space, and shows its 
peculiar crater-like top, while the negative carbon usually 
pierces but a small aperture. 

Paper reddened by the iodide of mercury had its colour dis¬ 
charged at the places on which the invisible image of the 
coal points fell upon it. 

Disks of paper reduced to carbon by different processes were 
raised to brilliant incandescence, both in the air and in the 
exhausted receiver. 

In these earlier experiments I turned to account apparatus 
which had been constructed for other purposes. The mirror 
employed, for example, was detached from a Duboscq’s camera, 
the ordinary silvering at the back being first made use of; the 
mirror being subsequently improved by silvering in front. The 
cell employed for the iodine solution was also that which 
usually accompanies Duboscq’s lamp, being intended by its 
maker for a solution of alum. Its sides are of good white 
glass, the width from side to side being 1*2 inch. 

A point of considerable theoretic importance was in¬ 
volved in these experiments. In his ‘excellent researches on 
fluorescence, Professor Stokes had invariably found the refran- 
gibility of the incident light to be lowered. This rule was so 
constant as almost to enforce the conviction that it was a law 
of nature. But if the rays which in the foregoing experiments 
raised platinum and gold and silver to a red heat were wholly 
ultra-red, the rendering visible of the metallic films would be 
an instance of raised refrangibility. 

And here I thought it desirable to make sure that no trace of 


292 


OX CALORESCEXCE, OR THE 


visible radiation passed through the solution, and also that the 
invisible radiation was exclusively ultra-red. 

This latter condition might seem to be unnecessary, because 
the calorific action of the ultra-violet rays is so exceedingly 
feeble (in fact so immeasurably small) that, even supposing 
them to reach the platinum, their heating power would be an 
utterly vanishing quantity. Still there were considerations 
which rendered the exclusion of all rays, of a higher refrangibility 
than those generated at the focus, necessary to the rigid solu¬ 
tion of the problem. Hence, though the iodine employed in 
the foregoing experiments was sufficient to cut off the light of 
the sun at noon, I wished to submit its opacity to a severer test. 
The following experiments were accordingly executed :— 

The iodine cell being placed in position, a piece of thick 
black paper, mounted on a retort-ring, was caused gradually to 
approach the focus of obscure rays. The position of the focus 
was announced by the piercing of the paper; the combustion 
being quenched, the retort-ring was moved slightly' nearer 
to the lamp, so that the converged beam passed through the 
burnt aperture, the focus falling about half an inch beyond it. 
A bit of blackened platinum held immediately behind the 
aperture was raised to redness over a considerable space. The 
platinum was then moved to and fro until the maximum degree 
of incandescence was obtained, the point where this occurred 
being accurately marked. A second cell containing a solution 
of alum, was then placed between the diaphragm of black paper 
and the iodine-cell. The alum solution diminished materially 
the invisible radiation, but it was without sensible influence on 
such visible rays as the concentrated beam contained. 

The two cells being in position, all stray light issuing from 
the crevices in the lamp was cut off; the daylight was also 
excluded from the room, and the eye being brought to a level 
with the aperture was slowly moved towards it, until the point 
which marked the focus was reached. A singular appearance 
presented itself. The incandescent coke points were seen per¬ 
fectly black, projected on a deep-red ground. When the points 
were moved up and down, their black images moved also. 
When brought into contact, a white space was seen at the 
extremities of the points, appearing to separate them. The 
points were seen erect. By careful observation the whole 


TRANSMUTATION OF HEAT RAYS. 


293 


of the carbon-rods could be seen, and even the holders which 
supported them. The darkness of the incandescent portion of 
the carbon-rods could of course onty be relative : they, in fact, 
intercepted more of the light reflected from the mirror behind 
than they could make good by their direct emission, hence their 
apparent blackness. 

The solution of iodine, 1*2 inch in thickness, proving unequal 
to the severe test here applied to it, I had two other cells con¬ 
structed—the one with transparent rock-salt sides, the other 
with glass ones. The width of the former was 2 inches, that of 
the latter nearly 2^ inches. Filled with the solution of iodine, 
these cells were placed in succession in front of the camera, and 
the concentrated beam was sent through them. Determining 
the focus as before, and afterwards introducing the alum-cell, 
the eye on being brought up to the focus received no impression 
of light. The alum-cell was then abandoned, and the unde¬ 
fended eye was caused to approach the focus : the heat was 
intolerable, but it seemed to affect the evelids and not the re- 
tina. An aperture somewhat larger than the pupil being made 
in a metal screen, the eye was placed behind it, and brought 
slowly and cautiously up to the focus. Nearly the whole con¬ 
centrated beam here entered the pupil; but no impression• of 
light was produced, nor was the retina sensibly affected bj the 
heat. The eye was then withdrawn, and a* plate of platinized 
platinum was placed in the position occupied by the retina a 
moment before. It instantly rose to vivid redness .* 

The rays which produced this incandescence were certainly 
invisible ones, and the subsequent failure to obtain, with the 
most sensitive media, and in the darkest room, the slightest 
evidence of fluorescence at the obscure focus, proved the in¬ 
visible rays to be exclusively ultra-red. 

§ 8. 

Improvement of Mirrors.—Exalted Effects of Combustion 

at Dark Focus. 

When intense effects are sought after, the problem is to col¬ 
lect as many of the invisible rays as possible, and to concentrate 

# 

* I do not recommend the repetition of these experiments. 


294 


ON CALORESCENCE, OR THE 


them on the smallest possible space. The nearer the mirror is 
to the source of rays, the more of these rays will it intercept and 
reflect; and the nearer the focns is to the same source, the 
smaller, and more intense, will the image be. To secure prox¬ 
imity both of focus and mirror, the latter must be of short 
focal length. If a mirror of long focal length be employed, its 
distance from the source of rays must be considerable to bring 
the focus near the source ; but when placed thus at a distance, 
a great number of rays escape the mirror altogether. If, on the 
other hand, the mirror be too deep, spherical aberration comes 
into play ; and, though a vast quantity of rays may be col¬ 
lected, their convergence is imperfect. 

To determine the best form of mirror, I had three constructed: 
the first is 4’1 inches in diameter, and of 1*4 inch focal length; 
the second 7*9 inches in diameter, and of 3 inches focal length ; 
the third 9 inches in diameter, with a focal length of 6 inches. 
Fractures caused by imperfect annealing repeatedly occurred; 
but at length I was fortunate enough to obtain the three 
mirrors, each without a flaw. The mirrors were all silvered in 
front, and thus the absorption due to the transmission of the heat 
through glass was avoided. The most convenient distance of 
the focus from the source I find to be about five inches ; and the 
position of the mirror ought to be arranged accordingly. This 
distance permits of the introduction of an iodine-cell of sufficient 
depth, while the heat at the focus is exceedingly powerful. 

The isolation of the luminiferous sether from the air is 
strikingly illustrated by these experiments. The air at the focus 
may be of a freezing temperature, while the sether possesses an 
amount of heat competent, if absorbed, to impart to that air the 
temperature of flame. An air-tliermometer is unaffected where 
platinum is raised to a white heat. Numerous experiments will 
suggest themselves to every one who wishes to operate upon 
the invisible heat-rays. Dense volumes of smoke rise from 
a blackened block of wood when it is placed in the dark 
focus: matches are of course at once ignited, and gunpowder 
instantly exploded. Dry black paper held there bursts into 
flame. Chips of wood are also inflamed, the dry wood of 
a hat-box being very suitable for this experiment. When a 
sheet of brown paper is placed a little beyond the focus, it is 
first brought to vivid incandescence over a large space; the 


TRANSMUTATION OF HEAT RAYS. 


295 


paper then yields, and the combustion prapagates itself as a 
burning ring round the centre of ignition. Charcoal is reduced 
to an ember at the focus, and disks of charred paper glow with 
extreme vividness. Sheet-lead and tin, if blackened, may be 
fused, while a thick cake of fusible metal is quickly pierced and 
melted. Blackened zinc-foil placed at the focus bursts into 
flame ; and by drawing the foil slowly through the focus, its 
ignition may be kept up till the whole of the foil is consumed. 
Magnesium wire, flattened at the end and blackened, also bursts 
into vivid combustion. A cigar or a tobacco-pipe may of course 
be instantly lighted at the dark focus. The bodies experimented 
on may be enclosed in glass receivers ; the concentrated rays 
will still burn them, after having crossed the glass. A small 
chip of wood in a jar of oxygen bursts • suddenly into flame : 
charcoal burns, while charcoal-bark throws out suddenly showers 
of scintillations. 

4 

§ 9. 

Transmutation of Heat Rays. — Calorescence. 

In all these cases the body exposed to the action of the 
invisible rays was more or less combustible. But it required 
to be heated to initiate the attack of the atmospheric oxygen. 
Its vividness was in great part due to combustion, and does 
not furnish a conclusive proof that the refrangibility of the 
incident rays was elevated. This, which is the result of greatest 
theoretic import, is effected by exposing non-combustible bodies 
at the focus, or by enclosing combustible ones in a space devoid 
of oxygen. Both in air and in vacuo platinised platinum-foil 
has been repeatedly raised to a white heat. The same result 
has been obtained with a sheet of charcoal or coke suspended in 
vacuo. On looking at the white-hot platinum through a prism 
of bisulphide of carbon, a rich and complete spectrum was ob¬ 
tained. All the colours, from red to violet, glowed with extreme 
vividness. The waves from which these colours were primarily 
extracted had neither the visible nor the ultra-violet rays com¬ 
mingled with them; they were exclusively ultra-red. The action 
of the atoms of platinum, copper, silver, and carbon upon these 
rays transmutes them from heat rays into light rays. They 
impinge upon the platinum at a certain rate; they return from 


296 


ON CALORESCENCE, OR THE 


it at a quicker rate. Their refrangibility is thus raised, the 
invisible being rendered visible. 

To express this transmutation of heat rays into others of 
higher refrangibility, I would propose the term calorescence. It 
harmonises well with the term 6 fluorescence 5 introduced by 
Professor Stokes, and is also suggestive of the character of the 
effects to which it is applied. The phrase e transmutation of 
rays, 5 introduced by Professor Challis,* covers both classes of 
effects. 


§ io. 

Various Modes of obtaining with the Electric-light Invisible Foci 

for Combustion and Calorescence. 

In the foregoing section arrangements are described which 
were made with a view of avoiding the danger incidental to the 
use of so inflammable a substance as the bisulphide of carbon. 
I have since thought of accomplishing this end in a simpler 
way, and thus facilitating the repetition of the experiments. 
The arrangement shown in Figure. 25 may be adopted with 
safety. 

A B C D is an outline of the camera; 

x y the silvered mirror within it; 

c the carbon points of the electric- light; 

o jp the aperture in front of the camera, through wdiich issues 
the beam reflected by the mirror x y. 

Let the distance of the mirror from the carbon points be such 
as to render the reflected beam slightly convergent. 

Fill a round glass flask with the solution of iodine, and 
place the flask in the path of the reflected beam at a safe distance 
from the lamp. The flask acts as a lens and as a filter at the 
same time, the bright rays are intercepted, and the dark ones 
are powerfully converged. F, fig. 25, represents such a flask; 
and at the focus formed a little beyond it combustion and 
calorescence may be produced. 

The following results have been obtained with a series of flasks 
of different dimensions, at a distance of 3-^ feet from the carbon 
points : — 


* Philosophical Magazine , S. 4, vol. xii. 521. 



TRANSMUTATION OF HEAT RAYS. 


297 


1. With a spherical flask, Of inches in diameter: platinum 
was raised to redness at the focus, and black paper inflamed. 

2. Ordinary Florence flask, 3f inches in diameter: platinum 
raised to bright redness over a large irregular space. Near the 
lamp, the effects obtained with this flask were very striking. 

Fig. 25. 



3. Small flask, 1*8 inch in diameter, not quite spherical: 
platinum rendered white-hot; paper immediately inflamed. 

4. A still smaller flask, 1*5 inch in diameter: effects very 
good ; about the same as the last. 

Fig. 26. 



5. The bulb of a pipette: effects striking, but not quite so 
brilliant as with the less regularly shaped small flasks. 

It follows, as a matter of course, that where platinum is heated 
to whiteness, the combustion of wood, charcoal, zinc, and mag¬ 
nesium may also be effected. 




































298 


ON CALORESCENCE, OR THE 


By the arrangement here described, platinum has been 
heated to redness at a distance of 22 feet from the source of 
the rays. 

The best of mirrors, however, scatters the rays more or less; 
and, by this scattering, the beam at a great distance from the 
lamp becomes much enfeebled. The effect is therefore intensified 
when the beam is caused to pass through a tube polished 
within, which prevents the lateral waste of radiant heat. Such 
a tube, placed in front of the camera, is represented at A B, 
fig. 26. The flask may be held against its end by the hand, 
or it may be permanently fixed there. With a battery of 
fifty cells, platinum may be raised to a white heat at the focus 
of the flask. 


Eig. 27. 



Again, instead of a flask filled with the opaque solution, let a 
glass or rock salt lens (L, fig. 27), 2*5 inches wide, and having 
a focal length of 3 inches, be placed in the path of the re¬ 
flected beam. The rays are converged; and at their point of 
convergence all the effects of calorescence and combustion may 
be obtained. 

In this case the luminous rays are to be cut off by a cell (m n ) 
with plane glass sides, which may be placed either before or 
behind the lens. 

Finally, the arrangement shown in* fig. 28 may be adopted. 
The beam reflected by the mirror within the camera is received 
and converged by a second mirror, x' y'. At the point of con¬ 
vergence, which may be several feet from the camera, all the 
effects hitherto described may be obtained. The light of the 
beam may be cut off at any convenient point of its course ; but 





















TRANSMUTATION OF HEAT RAYS. 


299 


in ordinary cases the experiment is best made by employing the 
bichloride instead of the bisulphide of carbon, and placing the 
cell (m n) containing the opaque solution close to the camera. 
The moment the coal points are ignited, explosion, combustion, 
or calorescence, as the case may be, occurs at the focus. 

The ordinary lamp and camera of Duboscq may be employed 
in these experiments. With proper mirrors, which are easily 
procured, a series of effects which, I venture to affirm, will in¬ 
terest everybody who witnesses them, may with the greatest 
facility be obtained. 

It is also manifest that, save for experiments made in dark¬ 
ness, the camera is not necessary. The mirrors and filter may 
be associated with the naked lamp. 


Fig. 28. 



I have sought several times to fuse platinum with the invi¬ 
sible rays of the electric-light, but hitherto without success. 
In some experiments a large model of Foucault’s lamp was 
employed, and a battery of 100 cells. In others I employed 
two batteries, one of 100 cells and one of 70, making use 
of two lamps, two mirrors, and two ray-filters, and converging 
the heat of both lamps upon opposite sides of the same film 
of platinum placed between them. The platinum was heated 
thereby to dazzling whiteness. I am persuaded that the metal 
could be fused, if the platinum-black could be retained upon its 
surface. But this being dissipated by the intense heat, the re- 
flecting-power of the metal comes into play, and lowers the 
absorption so much as to prevent fusion. By coating the plati¬ 
num with lampblack it has been brought to the verge of melting, 




























300 


ON (FLORESCENCE, OR THE 


the incipient yielding of the mass being perfectly apparent after 
it had cooled. Here, however, as in the case of the platinized 
platinum, the absorbing substance disappears too quickly. 
Copper and aluminium, however, when thus treated, are speedily 
burnt up. 


§ 11 . 


Invisible Foci of the Liw.e-light and the Sun . 

Thus far I have dealt exclusively with the invisible radiation 
of the electric-light; but all solid bodies raised to incandes¬ 
cence emit these invisible calorific rays. The denser the in¬ 
candescent body, moreover, the more powerful is its obscure 
radiation. We possess at the Royal Institution very dense 
cylinders of lime for the production of the Drummond light; 
and when a copious oxyhydrogen flame is projected against one 
of them it shines with an intense yellowish light, while the ob¬ 
scure radiation is exceedingly powerful. Filtering the latter 
from the total emission by the solution of iodine, all the effects 
of combustion and calorescence described in the foregoing: 
pages may be obtained at the invisible focus. The light ob¬ 
tained by projecting the oxy hydrogen flame upon compressed 
magnesia, after the manner of Signor Carlevaris, is whiter than 
that emitted by our lime; but the substance being light and 
spongy, its obscure radiation is surpassed by that of our more 

solid cylinders.* 

%> 

The invisible rays of the sun have also been transmuted. A 
concave mirror, 3 feet in diameter, was mounted on the roof of 
the Royal School of Mines in Jermyn Street. The focus was 
formed in a darkened chamber, in which the platinized platinum- 
foil was exposed. Cutting off the visible rays by the solution of 


* The discovery of fluorescence by Professor Stokes naturally excited speculation 
as to the possibility of a change of refrangibility in the opposite direction. Mr. Grove, 
I believe, made various experiments with a view to effect such a change ; but very soon 
after the publication of Professor Stokes’ memoir, Dr. Miller pointed to the lime-light 
itself as an instance of raised refrangibility. Prom its inability to penetrate glass 
screens, he inferred that the radiation of the oxyhydrogen flame was almost wholly 
ultra-red, an inference the truth of which has been since established by direct prismatic 
analysis. See Memoir VII. The intense light produced by the oxyhydrogen flame 
when projected upon lime must, he concluded, involve a change of period from slow 
to quick, or, in other words, a virtual elevation of refrangibility .—Elements of 
Chemistry , 1855, p. 210. 


TRANSMUTATION OF HEAT RAYS. 


301 


iodine, feeble but distinct incandescence was produced by tbe 
invisible rays. 

In a blackened tin tube (A B, fig*. 29), with square cross 
section and open at the end ? 

B, a plane mirror (x y ) was 
fixed, at an angle of 45° with 
the axis of the tube. A 
lateral aperture ( xo ), about 2 
inches square, was cut out 
of the tube, and over the 
aperture was placed a leaf of 
platinized platinum. Turning 
the leaf towards the concave 
mirror, the concentrated sun¬ 
beams were permitted to fall 
upon the platinum. In the 
glare of daylight it was quite 
impossible to see whether the platinum was incandescent or 
not; but on placing the eye at B, the glow of the metal could 
be seen by reflexion from the plane mirror. Incandescence 
was thus obtained at the focus of the large mirror, X Y, after 
the removal of the visible rays by the iodine solution, m n. 

To obtain a clearer sky, I had this mirror transferred to 
the garden of my friend, Mr. Lubbock, near Chislehurst. The 
effects obtained with the total solar radiation were extraordinary. 
Large spaces of the platinum-leaf, and even thick foil, when 
exposed at the focus, disappeared as if vaporized.* The handle 
of a pitchfork, similarly exposed, was soon burnt quite across. 
Paper placed at the focus burst into flame with almost explosive 
suddenness. The high ratio which the visible radiation of the 
sun bears to the invisible was strikingly manifested in these 
experiments. With a total radiation vastly inferior, the invi¬ 
sible rays of the electric-light, or of the lime-light, are competent 
to heat platinum to whiteness, while the most that could be 
obtained from the dark rays of the sun was a bright red calor- 
escence. The heat of the luminous rays, moreover, is so great 

* Concentrating the solar rays with a mirror 9 inches in diameter and of 6 inches 
focal length upon a leaf of platinized platinum, the metal was instantly pierced. 
Causing the focus to pass along the leaf, it was cut by the sunbeam as if a sharp in¬ 
strument had been drawn along it. 


Fig. 29. 





302 


ON CALORESCENCE, OR THE 


as to render it exceedingly difficult to work with the solution 
of iodine. It boiled up incessantly, exposure for two or three 
seconds being sufficient to raise it to ebullition. The high ratio 
of the luminous to the non-luminous radiation is doubtless to 
be ascribed in part to the absorption of a large portion of the 
latter by the aqueous vapour of the air. From it, however, may 
also be inferred the enormous temperature of the sun. 

Converging the sun’s rays with a hollow lens filled with the 
solution of iodine, calorescence was obtained on the roof of 
the Royal,Institution. 

Knowing the permeability of good glass to the solar rays, I 
requested Mr. May all to permit me to make a few experiments 
with his fine photographic lens at Brighton. Though exceed¬ 
ingly busy at the time, he in the kindest manner abandoned 
to my assistant, Mr. Barrett, the use of his apparatus for the 
three best hours of a bright summer’s day. A red heat was 
obtained at the focus of the lens after the complete withdrawal 
of the luminous portion of the radiation. 

§ 12 . 

Relation of Colour to Combustion by Dark Rays. 

Black paper has been very frequently employed in the fore¬ 
going experiments, the action of the invisible rays upon it 
being most energetic. This suggests that the absorption of those 
rays is not independent of colour. A red powder is red because 
of the entrance and extinction of the luminous rays of higher 
refrangibility than the red, and the ejection of the unabsorbed 
red light by reflexion at the limiting surfaces of the particles 
of the red body. This feeble absorption of the red extends 
to the rays of greater length beyond the red ; and the con¬ 
sequence is that red paper when exposed at the focus of in¬ 
visible rays is scarcely charred, while black paper bursts in a 
moment into flame. The following table exhibits the condition 
of paper of various kinds when exposed at the dark focus of an 
electric-light of moderate intensity:— 

Paper Condition. 

Glazed orange-coloured paper . Barely charred. 

„ red „ „ . Scarcely tinged; less than the orange. 

„ green ,, . Pierced with a small burning ring. 

„ ' blue „ „ . The same as the last. 


TRANSMUTATION OF HEAT RAYS. 


303 


Condition. 

. Pierced ; and immediately set ablaze. 

. Charred; not pierced. 

. Barely charred; less than the white. 

. Still less charred; about the same as the 
orange. 

. Scarcely tinged. 

. The same; a good deal of heat seems to 
get through these last two papers. 

. Pierced immediately; a burning ring ex¬ 
panding on all sides. 

. Pierced ; not so good as the last. 

. Pierced with a burning ring. 

. The same as the last. 

. Pierced; and immediately set ablaze. 

We have here an almost total absence of absorption on the 
part of the red paper. Even white absorbs more, and is conse¬ 
quently more easily charred. 

Rubbing the red iodide of mercury over paper, and exposing 
the reddened surface at the focus, a thermograph of the coal 
points is obtained, which discharges the colour at the place on 
which the invisible image falls. Expecting that this change of 
colour would be immediate, I was at first surprised at the time 
necessary to produce it. We are here reminded of Franklin’s 
experiments on cloths of different colours, and his conclusion 
that dark colours are the best absorbers. This conclusion might 
readily be pushed too far. Franklin’s colours were of a special 
kind, and their deportment by no means warrants a general con¬ 
clusion. The invisible rays of the sun possess, according to 
Muller, twice the energy of the visible ones. A white substance 
may absorb the former, while a dark substance—dark because of 
its absorption of the feeblest portion of the radiation—may not 
do so. The white powder of alum and the dark powder of iodine, 
exposed to the action of a source in which the invisible rays 
greatly surpass the visible in calorific power, exhibit a deport¬ 
ment at direct variance with the popular notion that dark colours 
are the best absorbers. 


Paper. 

Glazed black paper . 

„ white „ 

Thin foreign-post 
Foolscap . 

Thin white blotting-paper . 
,, whitey-brown „ 

Ordinary brown „ 

Thick brown „ 

Thick white sand-paper 
Brown emery „ 

Dead-black 


§ 13 . 

Calorescence through Bay-filters of Glass.—Remarks on the 

Black-bulb Thermometer. 

In conclusion, I would briefly refer to a few experiments 
made to determine the calorescence obtainable through glasses 


304 


ON CALORESCENCE, OR THE 


of various colours. In the first column of the subjoined table 
the colour of the glass is given ; in the second column the effect 
observed when a brilliant spectrum was regarded through the 
glass; and in the third column the appearance of a leaf of 
platinized platinum when placed at the focus, after the con¬ 
verging beam had passed through the glass :— 


Colour of Glass 
Dark red 
Mean red 

Light red 
Yellow . , . 

Green 


Dark purple . 
Mean purple . 
Light purple . 

Dark blue 


Mean blue 


Light blue 

Another blue glass 
Black glass No. 1 

Black glass No. 2 
Black glass No. 3 


{ 

/ 


Prismatic Examination 

Red only transmitted . 

Red only transmitted . 

Yellow intercepted with 
greatest power 

All the blue end absorbed . 

Besides the green, a dull red 
fringe and a blue band were 
transmitted 

Extreme blue and red trans¬ 
mitted 

Central portion of spectrum 
cut out 

Dims the whole spectrum, 
but chiefly absorbs the 
green 

Transmits the bhie, a green 
band, and a baud in the 
extreme red 

Blue; a yellowish-green band, 
and the extreme red trans¬ 
mitted 

' Transmits a series of bands— 
blue and green, a red band 
next orange, then a dark- 
red band, and finally ex¬ 
treme red 


f Dims all the spectrum: white 
* ^ light transmitted 

{ Whitish-green light trans¬ 
mitted 

. Deep-red light transmitted . 


Calorescence 
Dull white heat. 
White heat. 

Bright white. 

Vivid red with bright 
yellow in centre. 


No incandescence. 

0 

Vivid orange. 

Vivid orange. 

Vivid orango. 

Red heat. 

Reddish-pink heat. 


Pink heat, passing 
into red. 


Pink heat. 

Barely visible red. 

Dull red. 

Bright red, orange 
in the middle. 


The extremely remarkable fact here reveals itself, that when 
the beam of the electric lamp is sifted by certain blue glasses, 
the platinum at the focus glows with a distinct pink colour. 
Every care was taken to avoid subjective illusion here. The 
pink colour was also obtained at the focus of invisible rays. 
Withdrawing all the glasses, and filtering the beam by a 








TRANSMUTATION OF HEAT RAYS. 


305 


solution of iodine alone, platinum was heated nearly to white¬ 
ness at the focus. On introducing the pale blue glass between 
the iodine-cell and the focus, the calorescence of the platinum 
was greatly enfeebled—so much so that a darkened room was 
necessary to bring it out in full distinctness; when seen, how¬ 
ever, the thermograph was pink. A disk of carbonized paper 
being exposed at the obscure focus, rose at once to vivid white¬ 
ness when the blue glass was absent; but when present, the 
colour of the light emitted by the carbon was first a distinct 
pink; the attack of the atmospheric oxygen soon changes this 
colour, the combustion of the carbon extending on all sides as 
a white-hot circle.. If subsequent experiments should confirm 
this result, it would follow that there is a gap in the calorescence, 
the atoms of the platinum vibrating in red and blue periods, 
and not in intermediate ones. But I wish here to say that 
further experiments, which I hope shortly to make, are necessary 
to satisfy my own mind as to the cause of this phenomenon. 

The incandescent thermograph of the coal points being 
obtained on platinum, a very light-red glass introduced between 
the opaque solution and the platinum reduced the thermograph 
both in size and brilliancy. A second red glass, of deeper colour, 
rendered the thermograph still smaller and feebler. A dark-red 
glass reduced it still more—the visible surface being in this case 
extremely minute, and the heat a dull red merely. When, 
instead of the coloured glass, a sheet of pure white glass was 
introduced, the image of the coal points stamped upon the 
platinum-foil was scarcely diminished at all in brilliancy. A 
thick piece of glass of deep ruby-red proved equally trans¬ 
parent; its introduction scarcely changed the vividness of the 
thermograph. The colouring matter in this instance was the 
element gold, while the colour in the other red glasses was due 
to the compound suboxide of copper. Many specimens of gold- 
jelly, prepared by Mr. Faraday for his investigation of the colours 
of gold, though of a depth approaching to absolute blackness, 
showed themselves eminently transparent to the obscure heat- 
rays ; their introduction scarcely dimmed the brilliancy of the 
thermograph. Hence it would appear that even the metals them¬ 
selves, in certain states of aggregation, share that high diathermic 
poiuer which the elementary metalloids have been found to display . 

I have just said that a sheet of pure-white glass, when inter- 
20 


306 OX CALORESCEXCE, OR THE TRAXSMUTATIOX OF HEAT RAYS. 


posed in tli© path of the condensed invisible beam, scaicely 
dimmed the brilliancy of the thermograph. The intense calo¬ 
rific rays of the electric-light pass through such glass with 
freedom. We here come to a point of considerable practical 
importance to meteorologists. When such pure-white glass 
in a molten condition has carbon mixed with it, the re¬ 
sulting black glass is still eminently transparent to those 
invisible heat-rays which constitute the greater part of the 
sun’s radiation. I have pieces of glass, to all appearance black, 
which transmit 63 per cent, of the total heat of the electric 
light; and there is not the slightest doubt that, in thicknesses 
sufficient to quench entirely the light of the sun, such glasses 
would transmit a large portion of the invisible heat-rays. 
This is the glass often, if not uniformly, employed in the 
construction of our black-bulb thermometers, under the im¬ 
pression that the blackening secures the entire absorption of 
the solar rays. This conclusion is fallacious, and the instru¬ 
ments are correspondingly defective. A large portion of the 
sun’s rays pass through such black glass, impinge upon the 
mercury within the bulb, and are ejected by reflexion. Such 
rays contribute nothing to the heating of the thermometer. 

A sheet of common window-glass, apparently transparent, 
was placed between the iodine solution and the platinum- 
leaf at the focus; the thermograph was more dimmed than 
bv the black glass last referred to. The window-glass here 
employed, when looked at edgeways, was green ; and this 
experiment proves how powerfully this green colouring-matter, 
even in infinitesimal quantity, absorbs the invisible heat-rays. 
Perfect imperviousness might doubtless be secured by aug¬ 
menting the quantity of green colouring-matter. It is with 
glass of this description that the carbon should be mixed in the 
construction of black-bulb thermometers ; on entering such glass 
the solar rays would be entirely absorbed, and greater differences 
than those now observed would probably be found to exist 
between the black-bulb and the ordinary thermometer. 



IX. 


ON THE INFLUENCE OF COLOUR AND MECHANICAL 
CONDITION ON RADIANT HEAT. 


ANALYSIS OF MEMOIR IX. 


Franklin’s conclusion, that dark bodies absorb heat more greedily than light 
ones, glanced at in the last memoir (§ 12), is proved at the outset of this one 
to be by no means general. The dark powder of iodine is contrasted with the 
white powder of alum, and it is shown that the white body is by far the 
most powerful absorber. 

The fact of its being an element, and its proved diathermancy in solution, 
suggested iodine as a fitting substance to test the law of Franklin. 

The deportment of other elementary bodies is then glanced at, and two kinds 
of opacity are distinguished from each other—the one caused by a true absorp¬ 
tion, the other by the multiplied reflexions at the surfaces of particles not in 
optical contact. 

Thus ordinary sulphur stops the calorific rays, but not by absorption, and it 
is difficult therefore to burn it at the most intense focus. Were all its parts in 
optical contact, free transmission would follow. In pure crystallised sulphur 
this contact is attained, and the diathermancy of the substance is then remark¬ 
able. 

Black amorphous phosphorus, notwithstanding its inflammability, bears in¬ 
tense radiant heat for some time without ignition; ordinary phosphorus does 
the same. And this substance, which is dissolved in large quantities by bisul¬ 
phide of carbon, proves, when thus dissolved, almost perfectly diathermic. 

That substances which absorb radiant heat are those only which can be 
burnt by radiant heat, is further illustrated by pounded loaf-sugar when com¬ 
pared with table-salt. The one is immediately fused and ignited at the dark 
focus, the other is scarcely warmed. # 

It is also shown that a beam sifted by water, though its heat may be very 
great, has no power to melt the most delicate hoar-frost, and from this it is 
inferred that the melting of the Alpine snows and ice is the work not of the 
visible but of the invisible rays of the sun. 

Alcohol is shown to boil almost instantaneously at a focus where bisulphide 
of carbon is scarcely warmed. The errors on these points which have crept into 
physical treatises are shown to be due to the fact that a large portion of the 
sun’s emission consists of rays regarding which neither colour nor optical trans¬ 
parency gives us any information. 

Throughout these memoirs radiation and absorption have been looked upon 
as the acts of atoms and molecules ; chemical constitution, rather than physical 
condition, being regarded as the really potent agency. Still the numerous 
experiments of Masson and Courtepee seemed clearly to prove that all bodies, 
however different chemically, in a fine state of division possessed the same 
power of radiation and absorption. Melloni had also compared white and 
black substances, and found no difference in their radiative powers. 

It is shown, however, that in all those experiments it was not the powders, 


ANALYSIS OF MEMOIR IX. 


309 

but the varnish in which the powders were imbedded, that was really the sub- 
jec of experiment. Hence the observed equality. When the varnish is 
a andoned, differences in radiative and absorptive power immediately appear. 
ie powders here employed as radiators were first attached to a hot surface 
y sulphur cement; they were afterwards held there by electric attraction. 
Both methods showed the diversity existing among chemical precipitates as 
regards the radiation and absorption of heat. 

As regards colour, no general conclusion is possible: black, in some cases, 
transcends white; white, in other cases, excels black; while two blacks, or two 
whites, may differ greatly from each other. The use of glue, or varnish, to 
attach the powders to the hot surface abolished these differences. 

Thirty-two different powders held by sulphur cement are thus examined, and 
tound to vary in radiating power from 35-3 in the case of rock-salt, to 84 in the 
case of lampblack. Twenty-six powders held by electric attraction are examined, 

and found to vary from 24-5, in the case of rock-salt, to G5-8, in the case of black 
oxide of iron. 

The transmission by rock-salt of the heat emitted from twenty-four different 
substances is determined; and found to vary from 62-8, where rock-salt is itself 
the source of heat, to 89 where black platinum is the source. Looking at the 
absorptions by rock-salt instead of the transmissions, they are found to vary from 
3-7 with the platinum as source of heat to 29*9 with rock-salt itself as a source. 
These experiments leave no doubt upon the mind that the investigators were 

correct who affirmed rock-salt to be not equally diathermic to all kinds of 
heat. 

Expeiiments then follow illustrating the reciprocity of radiation and absorp¬ 
tion. 




IX. 


OX THE IXFLUEXCE OF COLOUR AXD MECHANICAL 
CONDITION ON RADIANT HEAT.* 

§ i. 

Pi oof that White Bodies sometimes absorb Heat more copiously 
* than Dark ones .— Explanation. 

Franklin placed cloths of various colours upon snow and 
allowed the sun to shine upon them. They absorbed the solar 
rays in different degrees, became differently heated, and sank 
therefore to different depths in the snow beneath them. His 
conclusion was that dark colours were the best absorbers, and 
light colours the worst; and to this hour we appear to have 
been content to accept Franklin’s generalization without quali¬ 
fication. In my last memoir I briefly pointed out its probable 
defects. Did the emission from luminous sources of heat consist 
exclusively of visible rays, we might fairly infer from the colour 
of a substance its capacity to absorb the heat of such sources. 
But the emission from luminous sources of heat is by no means 
all visible. In terrestrial sources of heat by far the greater 
part, and in the sun a very great part, of the emission consists 
of invisible rays, regarding which colour teaches us nothing. 

It remained therefore to examine whether the results of 
Franklin were the expression of a law of nature. Two cards 
were taken of the same size and texture; over one of them was 
shaken the white powder of alum, and over the other the dark 
powder of iodine. Placed before a glowing fire and permitted 
to assume the maximum temperature due to their position, it 
was found that the card bearing the alum became extremely 

* Received December 21st, 1865 ; and read before the Royal Society, January 18th, 
1866. Philosophical Transactions for 1866, page 83 ; Philosophical Magazine , October 
1866. 


312 


THE INFLUEXCE OF COLOUR 


hot, while that bearing the iodine remained cool. No thermo¬ 
meter was necessary to demonstrate this difference. Placing, 
for example, the back of the iodine card against the forehead 
or cheek, no inconvenience was experienced; while the back of 
the alum card similarly placed proved intolerably hot. 

This result was corroborated by the following experiments :— 
One bulb of a differential thermometer was covered with iodine, 
and the other with alum powder. A red-hot spatula being 
placed midway between both, the liquid column associated with 
the alum-covered bulb was immediately forced down, and main¬ 
tained in an inferior position. Again, two delicate mercurial 
thermometers had their bulbs coated, the one with iodine, the 
other with alum. On exposing them at the same distance to 
the radiation from a gas flame, the mercury of the alum-covered 
thermometer rose nearly twice as high as that of its neighbour. 
Two sheets of tin were coated, the one with alum, and the other 
with iodine powder. The sheets were placed parallel to each 
other, and about 10 inches asunder; at the back of each was 
soldered a little bar of bismuth, which with the tin plate to 
which it was attached constituted a thermo-electric couple. 
The two plates were connected together by a wire, and the free 
ends of the bismuth bars were connected with a galvanometer. 
Placing a red-hot ball midway between both, the calorific rays 
fell with the same intensity on the two sheets of tin, but the 
galvanometer immediately declared that the sheet which bore 
the alum was the most highly heated. 

In some of the foregoing cases the iodine was simply shaken 
through a muslin sieve; in other cases it was mixed with bisul- 
phide of carbon and applied with a cameTs-hair brush. When 
dried afterwards it was almost as black as soot; but as an ab¬ 
sorber of radiant heat it was no match for the perfectly white 
powder of alum. 

The difficulty of warming iodine by radiant heat is evidently 
due to the diathermic property which it manifests so strikingly 
when dissolved in bisulphide of carbon. The heat enters the 
powder, is reflected at the limiting surfaces of the particles, 
but it does not lodge itself among the atoms of the iodine. 
When shaken in sufficient quantity on a plate of rock-salt and 
placed in the path of a calorific beam, iodine cuts the latter off. 
But its opacity is mainly that of a white powder to light; it is 


AND mechanical condition on radiant heat. 313 


impervious, not through absorption , but through repeated internal 
reflexion. Ordinary roll sulplrur, even in thin cakes, allows no 
radiant heat to pass through it; but its opacity is also due to 
repeated internal reflexion. The temperature of ignition of 
sulphur is about 244° C.; but on placing a small piece of the 
substance at the focus of the electric lamp, where the tempera¬ 
ture was sufficient to heat platinum-foil in a moment to white¬ 
ness, it required exposure for a considerable time to fuse and 
ignite the sulphur. Though impervious to the heat, it was not 
adiathermic. The milk of sulphur was also ignited with some 
difficulty. Sugar is a much less inflammable substance than 
sulphur, but it is a far better absorber; exposed at the focus, it 
is speedily fused and burnt up. The heat, moreover, which is 
competent to inflame sugar is scarcely competent to warm 
table-salt. 

A fragment of almost black amorphous phosphorus was 
exposed at the dark focus of the electric lamp, but refused 
to be ignited. A still more remarkable result was obtained 
with ordinary phosphorus. A small fragment of this ex¬ 
ceedingly inflammable substance could be exposed for twenty 
seconds without ignition at a focus where platinum was almost 
instantaneously raised to a white heat. Placing a morsel of 
phosphorus on a plate of rock-salt and holding it before a 
glowing fire, it bears, as proved by my assistant, Mr. Barrett, 
an intense radiation without ignition; but laid upon a plate of 
glass and similarly exposed, the phosphorus soon fuses and 
ignites; its ignition, however, is not entirely due to radiant 
heat, but mainly to the heat imparted to it by the glass.* 

The fusing-point of phosphorus is about 44° C., that of sugar 
is 160°; still at the focus of the electric lamp the sugar fuses 
before the phosphorus. All this is due to the diathermancy of 
the phosphorus: a thin disk of the substance placed between 
two plates of rock-salt permits of a copious transmission. This 
substance therefore takes its place with other elementary bodies as 
regards deportment towards radiant heat. 

The more diathermic a body is, the less it is warmed by ra¬ 
diant heat, and no perfectly transparent body could be warmed 
by purely luminous heat. The surface of a vessel covered with 

* I believe this deportment of phosphorus towards radiant heat is not unknown to 
chemists. 


314 


THE INFLUENCE OF COLOUR 


a thick fur of hoar frost was exposed to the beam of the electric 
lamp condensed by a powerful mirror, the beam having been 
previously sent through a cell containing water; the sifted beam 
was powerless to remove the frost, though it was competent to set 
wood on fire. We may largely apply this result. It is not, for 
example, the luminous rays but the dark rays of the sun which 
sweep the snows of winter from the slopes of the Alps. Every 
glacier-stream that rushes through the Alpine valleys is almost 
wholly the product of invisible radiation. It is also the in¬ 
visible solar rays which lift the glaciers from the sea-level to 
the summits of the mountains; for the luminous rays penetrate 
the tropical ocean to great depths, while the non-luminous ones 
are absorbed close to the surface, and become the main agents 
in evaporation. 

It is often stated, without limitation, in chemical treatises 
that sulphuric ether may be exposed at the focus of a concave 
mirror without being sensibly heated; but this can only be 
true of a sifted beam. At the focus of the electric lamp, not 
only ether, but alcohol and water are speedily caused to boil, 
while bisulphide of carbon, whose boiling-point is only 48° C., 
cannot be raised to ebullition. In fact exposure for a period 
sufficient to boil alcohol or water is scarcely sufficient to render 
bisulphide of carbon sensibly warm. 


§ 2 . 

Melloni on Colours, and Masson and Courtepee on Powders, 

in relation to Radiant Heat. 

If any one point came out with more clearness than any 
other in my experiments on gases, liquids, and vapours, it was 
the paramount influence which chemical constitution exerted 
upon the phenomena of radiation and absorption. And, seeing 
how little the character of the radiation was affected by the 
change of a body from the state of vapour to the state of liquid, 
I held it to be exceedingly probable that even in the solid state 
chemical constitution would exert its power. But opposed to 
this conclusion we had the experiments of Melloni on chalk and 
lampblack, and the far more extensive ones of Masson and 
Courtepee on powders, which seemed clearly to show that in a 
state of extremely fine division, as in chemical precipitates, the 


AXD MECHANICAL CONDITION ON RADIANT HEAT. 315 

radiant and absorbent powers of all bodies are tbe same. From 
these experiments it was inferred that the influence of physical 
condition was so predominant as to cause that of chemical con¬ 
stitution to disappear.* 

A serious oversight, however, seems to have connected itself 
with all the experiments of these distinguished men. Melloni 
mixed his lampblack and powdered chalk with gum or glue, 
and applied them by means of a camel’s-hair brush on the 
surfaces of his radiating tube. Masson and Courtepee did the 
same. Melloni, it is true, thus compared a black surface with 
a white one; but the surfaces were seen to be white and black 
through the transparent gum, which in both cases was the real 
radiator. The same remark applies, to Masson and Courtepee. 
Every particle of the precipitates they employed was a varnished 
'particle ; and the constancy they observed was, I imagine, due 
to the fact that the main radiant in all their experiments was 
the substance employed to make their powders cling to the 
surfaces of their cubes. 


§ 3. 

New Experiments on Chemical Precipitates.—Influence of Colour 
and Chemical Constitution.—Sulphur Cement. 

Gum or glue is a powerful radiator—in fact, equal to lamp¬ 
black ; and it is a correspondingly powerful absorber; the par¬ 
ticles surrounded by it had therefore but small chance of radi¬ 
ating through it. I sought to remedy this by the employment 
of a diathermic cement. Sulphur is highly diathermic; it dis¬ 
solves freely in bisulphide of carbon; and at the suggestion of 
a chemical friend it was employed to fix the powders. The cube 
was laid upon its side, the surface to be coated being horizontal, 
and the bisulphide, containing the sulphur in solution, was 
poured over the surface. Before the liquid film had time to 
evaporate, the powder was shaken upon it through a muslin 
sieve. The bisulphide passed rapidly away in vapour, leaving 
the powder behind imbedded in the sulphur cement. Each 
powder, moreover, was laid on sufficiently thick to prevent the 
sulphur from surrounding its particles. This, though not per- 

* Masson and Courtepee, Comptes Rend us, vol. xxr. p. 938; Jarain, Cours de 
Physique, vol. ii. p. 289. 


316 


THE INFLUENCE OF COLOUR 


liaps a perfect way of determining the radiation of powders, 
was at all events an improvement on former methods, and 
yielded different results. 

Ten or twelve cubes of tin were employed in the investigation. 
One side of each of them was coated with milk of sulphur, and 
the fact that this substance was used throughout the entire series 
of cubes enabled me to connect all the results together. The 
cubes were heated with boiling water, and placed in succession 
at the same fixed distance in front of the thermo-electric pile, 
which as usual was well defended from air-currents and other 
extraneous sources of disturbance. Before giving the complete 
table of results, I will adduce a few of them, which show in a 
conclusive manner that, in solid bodies also, radiation is molecular 
rather than mechanical. 

The biniodide of mercury and the red oxide of lead resemble 
each other physically, both of them being of a brilliant red. 
Chemically, however, they are very different. Examined in the 
way indicated, their relative powers as radiators were found to 
be as follows :— 

Name Chemical Formula Radiation 

Biniodide of mercury . . . (Hg I 2 ) 39*7 

Red oxide of lead . . . . (2 Pb 0, Pb 0 2 ) 74-1 

Mixed with gum and applied with a cameTs-hair brush to 
the surfaces of the cube, the radiation from these two sub¬ 
stances fell out thus :— 

Name Radiation 

Biniodide of mercury ..... 80 

Red oxide of lead.80 

Here the influence of the gum entirely masks the difference 
due to molecular constitution. 

The effect of atomic complexity upon the radiation is well 
illustrated by the deportment of these two substances. It is 
further illustrated by the deportment of two different iodides of 
mercury:— 

Radiation 

Biniodide of mercury (Hg I 2 ) . . . . 39*7 

Iodide of mercury (Hg 2 I 2 ).46*G 

9 

Here the addition of a second atom of mercury to the 
molecule of the biniodide raises the radiation 7 per cent. The 




AND mechanical condition on radiant heat. 317 


experiment furnishes a kind of physical justification of the 
practice of chemists in regarding the molecule of yellow iodide 
of mercury to be Hg 2 I 2 , and not Hg I. 

The peroxide and protoxide of iron gave the following 
results:— 

Radiation 

Peroxide of iron.78-4 

Protoxide of iron.81-3 

I did not expect this, the protoxide being a less complex 
molecule than the peroxide. On examination, however, the 
protoxide was found to be in part the magnetic oxide. The 
formulae of the two substances are Fe 2 O 3 and Fe 0, Fe 2 O 3 , 
and the anomaly therefore disappears. 

Amorphous phosphorus and sulphide of iron gave the fol¬ 
lowing results:—• 

Radiation 

Amorphous phosphorus.63’6 

Sulphide of iron.81*7 

Sugar and salt were reduced in a mortar to the state of 
exceedingly fine powders. In point of cohesion and physical 
aspect these substances closely resemble each other; their 
radiative powers, however, are as follows:— 

Radiation 

Salt.35-3 

Sugar.70 

In his last interesting paper on emission at a red heat,* M. 
Desains mentions oxide of zinc as a body which at 100° C. has 
the same emissive power as lampblack. This is nearly true for 
the hydrated oxide; with the calcined oxide the following is 


fhe relation:— 

Radiation 

Lampblack ........ 84 

Hydrated oxide of zinc.80*4 

Calcined „ ....... 53'2 


Two red powders have been already compared together. 
With black platinum and black oxide of iron the following 
results were obtained :— 

Radiation 

Black platinum (electrolytic) . . . .59 

Black oxide of iron.8T3 


* Comptes Bcndus, July 3, 1865 ; Philosophical Magazine, August 1865. 






318 


THE INFLUENCE OF COLOUR 


The black platinum here employed was obtained by electro¬ 
lysis, a sheet of platinum-foil being coated with the substance. 

Chloride of silver and carbonate of zinc, two white powders, 
gave the following results :— 

Badiation 

Chloride of silver....... 32-5 

Carbonate of zinc.77*7 

When held upon its cube by the sulphur cement, the chloride 
of silver soon darkens in the diffuse light of the laboratory. It 
first becomes lavender, and passes through various stages of 
brown to black. During these changes, which may be asso¬ 
ciated with a chemical reaction between the chloride of silver 
and the sulphur in which it is embedded, the radiation steadily 
augments. Beginning in one instance with a radiation of 25, 
the chloride ended with a radiation of 60. 

We have thus far compared two red surfaces, two black 
surfaces, and two white surfaces together. The comparison of 
a black and white surface gave the following result:— 

Badiation 

Black platinum.59 

White hydrated oxide of zinc .... 80 - 4 

Here the radiation of the white body far transcends that from 
the black one. 

Comparing black and white a second time, we have the 
following result:— 

Badiation 

Oxide of cobalt.76-5 

Carbonate of zinc.77*7 

Here the black radiation is sensibly equal to the white one. 

Comparing black and white a third time, we have the 
following result:— 

Badiation 

Lampblack.84 

Chloride of lead.. 

Here the radiation of the black body far transcends that of 
the white one. 

We have thus compared red powders with red, black with 
black, white with white, and black with white; and the 
conclusion to be drawn is, I think, that chemical constitution, 


AND MECHANICAL CONDITION ON RADIANT HEAT. 319 

so^ far from being of vanishing value, is a really potent 
influence in the experiments. Combined with previous ones, 
they show that. the influence of chemical constitution makes 
itself felt in all states of aggregation—gaseous, liquid, and 

solid whether the solid be a chemical precipitate or a coherent 
mass. 

Were the radiative power of these substances determined by 
the state of division, it would probably make itself sensible even 
in a case where the division is effected by the pestle and mortar; 
but I do not find this to be the case. A plate of glass was fixed 
against the polished surface of a Leslie’s cube, and on the plate 
the powder of glass, rendered as fine as the pestle and mortar 
could make it, was strewn. It was caused to adhere without 
cement ot any kind. The cube was filled with boiling water and 
presented to the thermo-electric pile until a permanent deflec¬ 
tion was obtained. The cube being permitted to remain in its 
position, the powder was simply removed with a camel’s-hair 
biusli. The increase of radiation was only such as might be 
expected from the slight difference of temperature between the 
surface of the glass plate and the powder strewn upon it. Similar 
experiments, with precisely similar results, were made with a 
plate of rock-salt, oii which the finely divided powder of rock- 
salt was shaken. 

Still the same substance in different states of molecular 
aggregation may produce very different effects, both as regards 
radiation and absorption. We have already had an instance of 
this in the case of ozone as compared with ordinary oxygen. 
The following instance may also be cited:— 

One side of a Leslie’s cube was covered with a sheet of bright 
platinum-foil, and a second face by a similar sheet on which 
black platinum had been deposited by electrolysis. As radia¬ 
tors these two sheets of foil behaved in the following manner:_ 

Radiation 

Bright platinum-foil.6 

Platinized platinum.45*2 

Here the radiation of the black platinum is nearly eight times 
that of the bright substance. 


320 


THE INFLUENCE OF COLOUR 


§ 4 . 

Tabulation of the Radiant Powers of Powders.—Employment of 
Electric Attraction instead of Sulphur Cement. 

For the sake of reference, I will here tabulate the results 
obtained with a considerable number of precipitates when sub¬ 
jected to the described conditions of experiment. 


Table I.— Radiation from Poicders imbedded in Sulphur Cement. 


Name of Substance Radiation of 
Substance named 


Rock-salt 

353 

Biniodide of mercury 

397 

Milk of sulphur 

40-6 

Common salt . 

41-3 

Yellow iodide of mercury 

46-6 

Sulphide of mercury 

466 

Iodide of lead. 

47-3 

Chloride of lead 

55*4 

Chloride of cadmium 

56o 

Chloride of barium . 

5S-2 

Chloride of silver (dark). 

586 

Fluor-spar 

68-4 

Tersulphide of antimony. 

69-4 

Carbonate of lime . 

70-2 

Oxysulphide of antimony 

70-5 

Sulphide of calcium 

71 


Name of Substance Radiation of 
Substance named 

Sulphide of molybdenum 71‘3 

Sulphate of baryta . . 71‘6 

Chromate of lead . . 7-4*1 

Red oxide of. lead . . 74:2 

Sulphide of cadmium . 76-3 

Subchloride of copper . 76-5 

Oxide of cobalt . . 76'7 

Sulphate of lime . . 77'7 

Carbonate of zinc . . 77*7 

Red oxide of iron . . 78*4 

Sulphide of copper . .79 

Hydrated oxide of zinc . 80‘4 

Black oxide of iron . . 81 *3 

Sulphate of iron . . 81*7 

Iodide of copper . . 82 

Lampblack . . .84 


I subsequently endeavoured to get rid of the sulphur cement, 
and to make the powders adhere by wetting them with pure 
bisulphide of carbon, applying them to the cubes while wet. 
Some of the powders clung, others did not. My ingenious 
friend Mr. Duppa suggested to me that the powders might be 
held on by electrifying the cubes. I tried this plan, and found 
it simple and practicable. It was, however, aided by a circum¬ 
stance which we did not anticipate. The cube being placed 
upon an insulating stand with its side horizontal, the powder was 
shaken over it, and electrified by a few turns of a machine. It 
was found that the cube might then be discharged and set up¬ 
right, the powders clinging to it in this position. The results 
obtained with this arrangement are recorded in the following 
table :— 







AND MECHANICAL CONDITION ON RADIANT HEAT. 321 


Table II.— Radiation from 

Substance Radiation 

Rock-salt . . . 24-5 

Chloride of silver (white) 25 
Milk of sulphur . . 25 8 

• Biniodide of mercury . 26 

Iodide of lead. . . 36 

Sulphide of mercury , 30-6 

Spongy platinum . . 315 

\V ashed, sulphur (flowers) 32*3 
Sulphide of zinc . . 36-1 

Amorphous phosphorus . 38 
Chloride of lead . .39 

Chloride of cadmium . 40 

Fluor-spar . . . 48-6 


Powders held by Electricity. 


Substance 

Radiation 

Sulphide of calcium 

49-1 

Sulphate of baryta . 

51-3 

Sugar . ’ . 

52-1 

Red oxide of lead . 

56-5 

Sulphide of cadmium 

56-9 

Sulphate of lime 

59*3 

Chloride of silver (black) 

60 

Carbonate of zinc . 

62 

Oxide of cobalt 

62-5 

Iodide of copper 

63 

Red oxide of iron . 

63-8 

Sulphide of iron 

65-5 

Black oxide of iron . . 

65-8 


Tlie agreement as regards relative radiative power between 
this and the former table is as good as could under the circum¬ 
stances be expected. The experiments have been several times 
repeated ; and the table contains the means of the results, which 
were never widely different from each other. 


♦ 


§ 5 . 

Qualitative Experiments.—Radiation of various Bodies through 
Rock-salt .— Unequal Diathermancy of the Substance. 

The quantity of radiant heat emitted by bodies in all states 
of aggregation having been thus conclusively shown to depend 
mainly upon their molecular character, the question as to the 
quality of the heat emitted next arises. In examining this 
point, I contented myself with testing the heat radiated* from 
various substances by its transmission through rock-salt. The 
choice of this substance involved the solution of the still dis¬ 
puted question whether rock-salt is equally pervious to all kinds 
of rays.* For if it absorbed the radiation from two different 
bodies in different degrees, it would not only show a difference 
of quality in the radiations, but also demonstrate the incapacity 
of rock-salt to transmit equally rays of all descriptions. 

* The last publication on this subject is from the pen of that extremely able expe¬ 
rimenter Professor Knoblauch. After discussing the results of De la Provostaye and 
Desains, and of Mr. Balfour Stewart, he arrives at a different conclusion— namely, 
that pure rock-salt is equally pervious to all kinds of heat.— Poggendorff’s Annalen 
1863, vol. cxx. p. 177. 

21 





322 


THE INFLUENCE OF COLOUR 


The plate of salt chosen for this purpose was a very perfect 
one. I have never seen one more pellucid. The thickness was 
0*8 of an inch, and its size, compared with the aperture in front 
of which it was placed, was such as to prevent any part of the 
rays reflected from its lateral boundaries from mingling with 
the direct radiation. M. Knoblauch has clearly shown how the 
absence of caution in this particular may lead to error. The 
mode of experiment was that usually followed: the source of 
heat was first permitted to radiate against the pile, the de¬ 
flection produced by the total radiation being noted. The plate 
of rock-salt was then interposed ; the deflection sank, and from 
its new value the transmission through the rock-salt was calcu¬ 
lated and expressed in hundredths of the total radiation. 


Table III.— Transmission through ^Rock-salt of Heat radiated by 
the following Substances heated to 100° C. 


Substance 

% 

Rock-salt 

Biniodide of mercury 
Milk of sulphur 
Common salt 
Yellow iodide of mercury 
Sulphide of mercury . 
Iodide of lead . 

Chloride of lead 
Chloride of cadmium 
Chloride of barium . 
Chloride of silver (dark) 
Fluor-spar 

Tersulphide of antimony 
Carbonate of lime 
Oxysulphide of antimony 
Sulphide of molybdenum 
Sulphate of baryta . 
Chromate of lead 
Red oxide of lead 
Subchloride of copper 
Oxide of cobalt 
Red oxide of iron 
Sulphide of copper . 
Black oxide of iron . 
Sulphide of iron 
Lampblack 


Transmission in 
lOOtbsot the 
Total Radiation 

Total 

Radiation 

. 67*2 

35'3 . 

. 76-3* 

39-7 

. 76-9* 

40-6 

. 708 

41-3 

. 79* 

46-6 

. 731 

46-6 

. 73-8 

47-3 

. 73-1 

554 

. 73-2 

56-5 

. 70-7* 

58-2 

. 71-2 

586 

. 70-5* 

68-4 

. 77-1 

69-4 

. 77-6 

70-2 

. 77-6 

70-5 

. 78-4 

71-3 

. 71*3 

78-4 

. 71-6 

79-2 

. 74-1 

79-2 

. 76-3 

78-6 

. 76-5 

79 7 

— T 

CO 

81 

. 79 

82-3 

. 81-3 

827 

. 81-7 

83-3 

. 84 

83*3 


Here we have a transmission varying from 67 per cent, in the 







AXD MECHANICAL CONDITION ON RADIANT HEAT. 323 

case of powdered rock-salt to 84 per cent, in the case of lamp¬ 
black. The second column of figures will be referred to imme¬ 
diately. 

The powders here employed were fixed by the sulphur cement. 
The same powders held by electricity, and permitted to radiate 
through the rock-calt, gave the following transmissions :— 


Table IY. 


Substance Transmission 


Rock-salt 

628 

Chloride of silver (white) 

697 

Eluor-spar 

707 

Sulphide of mercury 

71 

Sulphide of calcium 

72o 

Milk of sulphur 

72-8 

Sulphide of cadmium 

73-3 

Biniodide of mercury 

737 

Washed sulphur 

74 

Iodide of lead 

74-1 

Sulphate of lime 

74-2 

Sulphide of sine 

74-4 


Substance Transmission 

Carbonate of zinc . 

74-8 

Sulphate of baryta . 

75 

Common sugar 

75-4 

Sulphide of copper . 

76-5 

Iodide of copper 

76-5 

Red oxide of iron . 

76-8 

Chloride of silver (black) 

77 3 

Amorphous phosphorus . 

78 

Oxide of cobalt 

78-2 

Sulphide of iron 

78-5 

Black oxide of iron . 

79 7 

Black platinum 

89 


The transmissions here are lower than when the sulphur 
cement was employed. I do not, however, think that the dif- 
, ferences are due to the employment of the cement, but to a 
slight source of disturbance, which was removed in the later 
experiments. 


It will be remarked that, as a general rule, powerful radiators 
have their heat more copiously transmitted by the rock-salt 
than feeble ones. To render this clear, in Table III., ap¬ 
pended to the transmission, is the corresponding total radiation. 
The only striking exceptions to the rule are marked with 
asterisks. This result, I think, is what might fairly be ex¬ 
pected ; for the peculiarity which enables one molecule to radiate 
more heat than another, may also be expected to introduce dis¬ 
sonance between their rates of oscillation. The probability is 
^therefore that greater dissonance will exist between the vibrating 
periods of good radiators and bad radiators than between the 
periods of the members of either class. But the greater the 
dissonance the less will be the absorption; hence, as regards 
transmission through rock-salt, we have reason to expect that 
powerful radiators will find a more open door to their emission 





324 


THE INFLUENCE OF COLOUR 


than feeble ones. This is, as I have said, in general the case. 
But the rule is not without its exceptions; and the most striking 
of these is the case of black platinum, which, though but a 
moderate radiator, sends a greater proportion of heat through 
rock-salt than any other known substance. 

In his latest investigation, Knoblauch examined at great 
length the diathermancy of rock-salt. With his usual acuteness, 
he points out several possible sources of error, and with his 
customary skill he neutralizes these sources. His conclusion is 
the same as that of Melloni, namely, that rock-salt transmits in 
the same proportion all sorts of rays. On the opposite side we 
find the experiments of MM. De la Provostaye and Desains, and 
those of Mr. Balfour Stewart,* both of which are discussed by 
Knoblauch. He differs from those experimenters, while my 
results bear them out. Considering the slow augmentation of 
transmission which the foregoing tables reveal, and the consi¬ 
derable number of bodies whose heat is transmitted in almost 
the same proportion by rock-salt, it is easy to see that, where 
the number of radiants is restricted, such a uniformity of trans¬ 
mission might manifest itself as would lead to the conclusion of 
Melloni and Knoblauch. It was only by the deliberate selection 
and extension of the substances chosen as radiators that the 
differences were brought out with the fulness and distinctness 
recorded in the foregoing tables. 

The differences in point of quality and the absence of perfect 
diathermancy in rock-salt appear more striking when instead of 
the transmissions we take the absorptions. In the case of the 
radiation from powdered rock-salt, for example, 37*2 per cent, of 
the whole radiation is intercepted by the rock-salt plate. Ac¬ 
cording to Melloni, between 7 and 8 per cent, of this is lost by 
reflexion at the two surfaces of the salt. This would leave in 
round numbers a true absorption of 30 per cent, by the plate of 
rock-salt. In the case of black platinum, the absorption simi¬ 
larly deduced amounts to only 4 per cent, of the total radiation. 
Instead, therefore, of the radiation from those two sources of 
heat being absorbed in the same proportion, the ratio in the one 
case is more than seven times that in the other. For the sake 

* I think the important experiment first executed by Mr. "Balfour Stewart, of rock- 
salt radiating through rock-salt, is of itself sufficient to demonstrate in the most un¬ 
equivocal manner that this substance is not equally pervious to all kinds of rays. 


AND MECHANICAL CONDITION ON RADIANT HEAT. 325 

of illustration, I here introduce a few of the absorptions deter¬ 
mined in this way :— 

Table V.— Radiation through Rock-salt. 

Source of heat 
Black platinum 
Black oxide of iron 
Red oxide of iron 
Sugar . 

Chloride of silver . 

Rock-salt 

These differences are so great as to enable every experimenter 
to satisfy himself with the utmost ease as to the unequal 
peimeability of rock-salt 5 and this facility of demonstration 
will, I trust, contribute to make inquirers unanimous 011 this 
important point. 


Absorption per 100 

. 37 

. 13 

. 15-9 

. 17-3 

. 22-6 
. 299 


§ 6 . 

Radiation of Powders.—Reciprocity of Radiation and Absorption. 


Theory alone would lead us to the conclusion tha t the absorp¬ 
tive power of the substances mentioned in Table I. is proportional 
to their radiative power ; nevertheless a few actual experiments 
on absorption will serve as a check upon those recorded in the 
table. These were conducted in the 
following manner :—A B (fig. 30) is 
a sheet of common block tin, 5 
inches high and 4 wide, fixed upon a 
suitable stand. At the back of A B 
is soldered one end of the small bar 
of bismuth 6 , the remainder of the 
bar, to its free end, being kept out 
of contact with the tin by a bit 
of cardboard. To the free end of 
b is soldered a wire which can be 
connected with a galvanometer. 

A' B' is a second plate of tin fur¬ 
nished also with its bismuth bar, and 
in every respect similar to A B. From one plate to the other 
stretches the wire W. C is a cube containing boiling water, 
placed midway between the two plates of metal. 


Fig. 30. 













































326 


THE INFLUENCE OF COLOUR 


The tin plates were in the first instance coated uniformly with 
lampblack, and the two surfaces of the cube which radiated 
against the plates were similarly coated. The rays from C being 
emitted equally right and left, and absorbed equally by the two 
coated plates A B and A x IT, warmed these plates to the same de¬ 
gree. It is manifest from the arrangement that, if the thermo¬ 
electric junctions were equally sensitive, the current generated 
at the one ought exactly to neutralize the current from the 
other. This was found to be very nearly the case. It is dif¬ 
ficult to make both junctions of absolutely the same sensitive¬ 
ness ; but the moving of the feebler plate a hair’s breadth nearer 
to the cube C enabled it to neutralize exactly the radiation from 
its opposite neighbour. My object now was to compare the 
lampblack coating of the plate A B with a series of other 
coatings, placed in succession on the other plate. These latter 
coatings were the powders already employed, and they were 
held upon A' B' by their own adhesion. 

When A B was coated with lampblack and A' B' with rock- 
salt powder, the equilibrium observed when both the plates were 
coated with lampblack did not exist. The lampblack, by its 
greater absorption, heated its bismuth junction most, and a 
permanent deflection of 59° in favour of the lampblack was 
obtained. Other powders were then substituted for the rock-salt, 
and the difference between them and the lampblack was deter¬ 
mined in the same way. When, for example, sulphide of iron 
was employed, there was a deflection of 30° in favour of lamp¬ 
black. The results obtained with six different powders thus 
compared with lampblack are given in the following table:— 


Table VI. 

Excess of lampblack above rock-salt . 

,, ,, fluor-spar . 

,, „ red lead 

,, ,, oxide of cobalt . 

„ „ sulphide of iron 


59 = 112 units. 


46 = 68 
40 = 45 
57 = 42 
30 = 30 


The order of absorption here coincides with the order of 
radiation of the same substances shown in Table III. 

But we can go further than the mere order of absorption. 
Removing the opposing plate, connecting the wire W direct 



AND MECHANICAL CONDITION ON RADIANT HEAT. 327 

with the galvanometer, and allowing the standard lampblack to 
exert its full action, the deflection observed was 

65° = 163 units. 

The numbers in Table YI. show us the excess of the lampblack 
over the substances there employed,—its excess in the case 
of rock-salt, a bad absorber, is 112, its excess in the case of 
sulphide of iron being only 30. Deducting, therefore, the 
numbers given in Table YI. from 163, the total absorption of 
lampblack, we obtain a series of numbers which expresses the 
absorption of the other substances. This series stands as 
follows:— 

Table YII. 


Substance 

Relative Absorptions 

Radiation from 

• 

< - 

- \ 

Table 11. 

Rock-salt. 

. 51 

25*5 

25 

Fluor-spar 

. 95 

47-5 

49 

Red lead . 

. 118 

59 

57 

Oxide of cobalt 

. 121 

60-5 

63 

Sulphide of iron 

. 133 

66'd 

66 

The first column of 

figures expresses 

the relative absorptions 


for the sake of comparison with the corresponding radiations, I 
have placed the halves of these numbers in the second column 
of figures, and in the third column the radiations obtained from 
Table II. The approximation of the figures in the second and 
third columns is close enough to establish the accurate pro¬ 
portionality of radiation and absorption. 

. . Throughout some of these investigations I have been effi- 
’ ciently assisted by Mr. Robert Chapman, and throughout others, 
with great skill and assiduity, by Mr. W. F. Barrett. 







X. 

ON THE ACTION OF RAYS OF HIGH REFRANGIBILITY 

UPON GASEOUS MATTER. 



ANALYSIS OF MEMOIR X. 


In the researches thus far placed before the reader rays of low refrangibility 
were invoked as the explorers of molecular condition. The present memoir 
deals with the interaction of rays of high refrangibility and gaseous matter. 

Theoretic considerations regarding atoms and molecules, and their relation 
to the waves of {ether, preface the memoir, and the special origin of the inquiry 
is indicated. The obstacles to be overcome are referred to, one of these being 
the floating matter of the air. 

This was found competent to pass through tubes containing caustic potash 
and sulphuric acid, thus showing the insecurity of experiments based on the 
assumption that these substances destroy the floating matter of the air. 

The vapours of various substances inclosed in a glass experimental tube are 
subjected to the action of a concentrated beam of light. The vapours are 
decomposed ; non-volatile products are formed which are precipitated as actinic 
clouds in the experimental tube. 

The nitrite of amyl, the iodide of allyl, and the iodide of isopropyl are 
adduced as examples of this new action of light upon vapours. 

In Memoir VI. it was shown that the order of absorption in vapours and 
their liquids is the same. So constant was this relation that in the various 
singular shiftings of diathermic position, revealed in the memoir referred to, 
the vapours followed with undeviating precision the fluctuations of their 
liquids. 

In the present memoir a similar relation is shown to hold good with rays of 
high refrangibility. The chemical 'penetrability of nitrite of amyl, both in the 
liquid and vaporous condition, is contrasted with that of the iodide of allyl. 
A layer one-eighth of an inch thick of the liquid nitrite suffices to remove the 
rays which act upon the nitrite vapour; while a foot or so of the nitrate vapour 
almost accomplishes the same thing. On the other hand, a whole inch of the 
liquid iodide of allyl is found incompetent to stop the rays which act upon the 
iodide, and six or seven feet of the iodide vapour are also found incompetent 
to accomplish this. 

The influence of a second body on the decomposition of vapours by light is 
illustrated ; and it is compared with the influence of chlorophyl on the decom¬ 
position of carbonic acid by the rays of the sun. 

The character of the actinic clouds produced, when the quantities of vapour 
acted on are very small, is described and illustrated. In all cases the cloud 
commences with a beautiful azure, which discharges perfectly polarized light 
at right angles to the direction of the beam. 

The identity of deportment of this azure with the blue of the firmament 
justifies the conclusion that the physical origins of both are identical: that in 
the experimental tube we have, to all intents and purposes, an artificial sky. 
Section 9 contains a condensed description of this portion of the inquiry; and 


ANALYSIS OF MEMOIR X. 331 

Section 10 embraces additional illustrations of the production of the firmamental 
blue. 

Section 11 describes the difficulty which beset the earlier stages of tho 
inquiry, and the extreme liability to error arising out of the action of infini¬ 
tesimal residues of active vapours. It is shown that an extraordinary amount 
of light may be scattered by cloudy matter of almost infinite tenuity, the 
phenomena of comets’ tails being thus rendered the subjects of experimental 
illustration. 

I may add here that cometary envelopes may be imitated with perfect accuracy 
by these actinic clouds. Motions of different angular magnitudes round the 
axis of the experimental tube always set in, and by the sliding of the layers of 
cloud over each other envelopes are formed, the interior ones being plainly 
seen through the exterior ones. As many as five such envelopes are frequently 
produced. May not the envelopes of comets be formed in a similar wav ? 
Differential motions started by differences of temperature would be undoubtedly 
competent to distribute the cometary matter in the layers or envelopes which 
observation reveals. Cometary envelopes according to this hypothesis would 
be the result of convection currents ; and these again would intimate that the 
visible matter floats in an invisible gas or vapour. 

Eight different vapours mixed in various proportions with air and hydrochloric 
acid and nitric acid, are subjected to examination, the resulting phenomena 
being described in detail. 

Finally, the vapours are subjected to the powerful dark foci obtained by the 
use of an iodine-filter; but the ultra-red rays are found totally incompetent 
to produce decomposition. In these cases, therefore, as in so many others, the 
rays of high refrangibility are the chemical rays. 


















































































































































X. 


ON THE ACTION OF RATS OF HIGH REFRANGIBILITY 

UPON GASEOUS MATTER* 

§-!• 

Introduction. 

Within the last ten years I have had the honour of sub¬ 
mitting to the Royal Society a series of investigations the 
principal aim of which was to render the less refrangible" rays 
of the spectrum interpreters and expositors of the molecular 
condition of matter. 

Unlike the beautiful researches of Melloni and Knoblauch, 
these inquiries made radiant heat a means to an end. My 
thoughts were fixed on it in relation to the matter through 
which it passed. Placing before my mind such images of 
molecules and their constituents as modern science justifies or 
renders probable, such images of the luminiferous aether and its 
motions as the undulatory theory enables us to form, I endea¬ 
voured to fashion and execute experiments founded upon these 
conceptions which should give us a surer hold upon molecular 
constitution. 

Thus definite physical ideas have accompanied and guided * 
the whole course of these researches. That matter is constituted 
of atoms and molecules has been accepted as a verity through¬ 
out. The phenomena under examination rendered it impossible 
for me to halt at the law of multiple proportions, which so 
many chemists of the present day appear inclined to make their 
intellectual bourne. In following up a train of gether waves, 
in idea, to their source, I could not place at that source a mul¬ 
tiple proportion ; for the waves could not be connected physi¬ 
cally with such a multiple. I was forced to put there a bit of 

* Received December 4, 1869 ; read before the Royal Society, January 27, 1870. 

."Philosophical Transactions for 1870, p. 333. 


334 THE ACTION OF BAYS OF HIGH REFRAXGIBILITY 

matter—in other words, a molecule —bearing the same relation 
to the sether as a vibrating string does to the air, which accepts 
its motion and transmits them as waves of sound. 

One result among many others which these researches estab¬ 
lished will probably play an important part in the chemistry of 
the future. I refer to the proved change of relation between 
the luminiferous sether and ordinary matter which accompanies 
the act of chemical combination. Here, without any alteration 
in the quantity, or in the ultimate quality of the medium 
traversed by the sethereal waves, vast changes occur in the 
amount of wave-motion intercepted. Let pure nitrogen and 
ordinary oxygen be mixed mechanically together in the pro¬ 
portion by weight of 14: 8. Radiant heat, it is now known, 
will pass through the mixture as through a vacuum. Ho doubt 
a certain amount of heat is intercepted; but it is so small 
an amount as to be practically insensible. At all events it is 
multiplied by hundreds, if not by thousands, the moment the 
oxygen and nitrogen combine to form nitrous oxide. Or let 
nitrogen and hydrogen be mixed mechanically together in the 
proportion of 14:3; the amount of radiant heat which they 
then absorb is augmented more than a thousandfold* the 
moment they build themselves together into the molecules of 
ammonia. Neither the quantity nor the ultimate quality of the 
matter is here changed; the act of chemical union is the sole 
cause of the enormous alteration in the amount of heat inter¬ 
cepted. The converse of these statements is of course also 
true; dissolve the chemical bond, either of the nitrous oxide or 
of the ammonia, and you instantly destroy the absorption. As 
a proof that our atmosphere is a mixture, and not a compound, 
no experiment with which I am acquainted matches in point of 
conclusiveness that which demonstrates the deportment of dry 
air to radiant heat. 

o c es which can thus intercept the waves of 
sether must be shaken by those waves, possibly shaken asunder. 
That ordinary thermometric heat provokes chemical actions is 
one of the commonest facts of observation. These actions, con¬ 
sidered from a physical point of view, are changes of molecular 
position and arrangement consequent on the acceptance of 

* It may be a millionfold; for we do not yet know how small the absorption of the 
absolutely pure mixture really is. 


UPOX GASEOUS MATTER. 


335 


motion from the source of heat. Radiant heat also, if suffi¬ 
ciently intense, and if absorbed with sufficient avidity, could 
produce all the effects of ordinary thermometric heat. The 
daik rays, for example, which can make platinum white hot, 
could also, if absorbed, produce the chemical effects of white- 
hot platinum. They could decompose w r ater, as now in a 
moment they can boil water. But the decomposition in this 
case would be effected by the virtual conversion of the radiant 
heat into tliermometric heat. There would be nothing in the 
act characteristic of radiation , or demanding it as an essential 
element in the decomposition. 

The dark calorific rays are powerfully absorbed by various 
bodies, but, as a general rule, they do not appear competent to 
set up that particular motion among the constituents of a 
molecule which breaks the tie of chemical affinity. All the 
rays of the spectrum exercise no doubt chemical powers. We 
should have scant vegetation upon the earth’s surface if the 
red and ultra-red rays of the sun were abolished. But the 
chemical actions in which the radiant form comes into play 
are mainly produced by the least energetic rays of the spectrum. 
The photographer has his heat focus in advance of the chemical 
focus y which latter, though potent for his special purposes, 
possesses almost infinitely less mechanical energy than its 
neighbour. Some special relation must, therefore, as a general 
rule, subsist between chemical molecules and the more refran¬ 
gible rays; we arrive at the conclusion, that chemical decompo¬ 
sition by rays, to keep to the ordinary term, is less a matter of 
amplitude on the part of the vibrating aether particles than of 
time of vibration. 

The decomposition of a molecule must result from the in¬ 
ternal strain of its atoms ; to the atoms, therefore, and not to 
the molecule as a whole, the vibrations which produce chemical 
change must be imparted. The question remains an out¬ 
standing one in molecular physics, why it is that the longer and 
more powerful aether waves are generally incompetent to set up 
the motion which results in decomposition. The influence of 
synchronism here suggests itself. These shorter waves are 
effectual because their motion is stored up. Their infinitesimal 
impulses, because imparted at the proper intervals, accumulate 
and finally become intense enough to jerk asunder the atoms 
wfith whose periods they are in accordance. 


336 THE ACTION OF EATS OF HIGH KEFEAXGIBILITY 


§ 2 . 


Theoretic Notions: Formation of Actinic Clouds through the 
Decomposition of Vapours by Light. 

The present investigation is in a certain sense complementary 
to those referred to at the outset of this paper. It deals with 
the relations of gaseous matter to the most refrangible rays of 
the spectrum. It treats of the chemical energies of such rays 
as exerted upon such matter. If we except the combination of 
chlorine and hydrogen by light, and the decomposition of car¬ 
bonic acid by the solar rays in the leaves of plants, which 
latter, however, may not be the decomposition of a gas, no 
fact, I believe, has hitherto been known to exist in which light, 
or heat in the radiant form, acts chemically upon a gas or 
vapour.* By this inquiry the range of radiant energy as 
a chemical agent is, therefore, considerably extended; the 
phenomena resulting from that energy are exhibited in a new 
and exceedingly impressive form, and they prompt reflexions 
regarding the possible influence of solar radiation on the gases, 
vapours, and effluvia of our atmosphere which could not pre¬ 
viously be entertained. 

The inquiry was started thus:—It is known to the Society 
that the experiments on radiant heat already referred to, were 
for the most part performed in tubes of brass or glass, called, 
for the sake of distinction, ‘experimental tubes.’ It is also 
known that a difference exists between my eminent friend, 
Professor Magnus,f and myself, with regard to the deportment 
of aqueous vapour towards radiant heat. Last autumn, and 
in reference to the reasons assigned by him for this difference, 
I scrutinised, by means of a powerful beam of light, the appear¬ 
ance of my experimental tubes during the entrance into them 
of various gases and vapours. The vapours were carried into 
the tubes by dry air, which had been permitted to bubble 
through their liquids. I watched carefully, and with the aid 
of magnifying-lenses, for any signs of the precipitation of mois¬ 
ture either upon the surface of the experimental tube itself, or 
upon the plates of rock-salt employed to close it, keeping at 

* Professor Stokes reminds me that Phosgen gas derives its name from its formation 
tinder the influence of light.—[J. T., July 1870.] 

f Unhappily lost to science since these words were written.—[J. T., July 1870.] 


UPON GASEOUS MATTER. 337 

the same time my mind open to any other action which the 
intensely concentrated luminous beam might reveal. ' 

On October 9, 1868, while thus engaged upon the vapour of 
the nitrite of amyl, I observed a curious cloudiness in the 
experimental tube when the beam was sent through it. For a 
moment this appearance troubled me; for it required a little 
reflexion to assure me that in my previous publications actions 
had not been sometimes ascribed to pure cloudless vapour 
which were really due to such nebulous matter as was then 
before me. The appearance, however, immediately declared 
itself to my mind as a product of chemical action then and 
there exerted on the vapour. 

The nitrite vapour was then intentionally subjected to >the 
action of the light. The beam employed was convergent. As 
the mixture of air and vapour reached the point of greatest 
concentration of the beam, cloudy matter was there precipitated, 
which was afterwards whirled by the moving air into the more 
distant parts of the tube. The cloud thus carried away was 
incessantly renewed, and after the mixed air and vapour had 
ceased to enter, precipitation occurred all along the cone of 
rays in front of the focus. 

^ The lamp was then extinguished, and the mixture of air and 
nitrite vapour permitted to enter the tube in the dark. When 
the tube was full, the condensed beam was sent through it. 
For a moment the light seemed to pass through a vacuum; but 
after a moment’s pause a white cloud fell suddenly upon the 
conical portion of the beam, causing it to flash forth almost 
like an illuminated solid. 

When the beam, previous to allowing it to enter the vapour, 
was caused to pass through a red or yellow glass, the action 
though visible was feeble; it was much, more energetic when 
the beam passed through a blue glass. A convergent beam 
was sent through a red glass, and the feeble effect was observed. 
A blue glass was then added, and by the concert of both the light 
was completely cut off. On withdrawing the red glass, a very 
beautiful blue cloud came down upon the conical beam. The 
experiment proved that in this case, as in so many others, the 
blue rays are the 6 chemical rays.’ 

Solar light, as might be expected, produces all the effects of 

the electric-light, and in regions more favoured than London 
22 


338 THE ACTION OF RAYS OF HIGH EEFRANGIBILITY 

may be employed in continuous researches of this nature. 
When the.parallel beams of the sun are duly concentrated, the 
precipitation which they invoke in passing through nitrite-of- 
amyl vapo'ur is copious and immediate. 

§ 3. 

Description of Apparatus. 

The simple apparatus employed in these experiments will be 
at once understood by reference to Figure 31. SS' is the glass 
experimental tube, which has varied in length from 1 to 5 feet, 
and which may be from 2 to 3 inches in diameter. From the 
end S the pipe pp' passes to an air-pump. Connected with 
the other end we have the flask F, containing the liquid whose 
vapour is to be examined; then follows a TJ-tube, T, filled with 
fragments of clean glass wetted with sulphuric acid ; then a 
second IJ-tube, T', containing fragments of marble wetted with 
caustic potash; and finally a narrow straight tube 11', contain¬ 
ing a tolerably tightly-fitting plug of cotton-wool. To save the 
air-pump gauge from the attack of such vapours as act on mer¬ 
cury, as also to facilitate observation, a separate barometer 
tube was employed. 

Through the cork which stops the flask F, two glass tubes, 
a and b, pass air-tight. The tube a ends immediately under 
the cork; the tube b, on the contrary, descends to the bottom 
of the flask and dips into the liquid. The end of the tube b is 
drawn out so as to render very small the orifice through which 
the air escapes into the liquid. 

The experimental tube S S' being exhausted, a cock at the 
end S' is carefully turned on. The air passes slowly through 
the cotton-wool, the caustic potash, and the sulphuric acid in 
succession. Thus purified it enters the flask F, and bubbles 
through the liquid. Charged with vapour it finally passes into 
the experimental tube, where it is submitted to examination. 
The electric lamp L, placed at the end of the experimental 
tube, furnished the necessary beam. 

§ 4 . 

The Floating Matter of the Air. 

Prior, to the discovery of the foregoing action, and also 
during the experiments just referred to, the nature of my 


Fig. 31. 


UPCXN- GASEOUS JITTER. 


339 


work compelled me to aim at obtaining experimental tubes 



absolutely clean upon tlie surface, and absolutely empty 












































































































































340 THE ACTION OF KAYS OF HIGH KEFKAXGIBILITY 


within. Neither condition is, however, easily attained. For 
however well the tubes might be washed and polished, and 
however bright and pure they might appear in ordinary 
daylight, the electric beam infallibly revealed signs and tokens 
of dirt. The air was always present, and it was sure to deposit 
some impurity. All ordinary chemical processes are open to 
this disturbance. When the experimental tube was exhausted 
it exhibited no trace of floating matter, but on admitting the air 
through the U-tubes containing caustic potash and sulphuric 
acid, a dust-cone more or less distinct was always revealed by 
the powerfully condensed electric beam. 

The floating motes resembled minute particles of liquid which 
might have been carried mechanically from the drying apparatus 
into the experimental tube. Precautions were therefore taken 
to prevent any such transfer, but with little or no mitigation. 
I did not imagine that the dust of the external air could 
find such free passage through the caustic potash and the 
sulphuric-acid tubes. But the motes really came from without. 
They also passed with freedom through a variety of ethers and 
alcohols placed in the flask F. In fact, it requires long- 
continued action on the part of an acid first to wet the motes 
and afterwards to destroy them. By carefully passing the air 
through the flame of a spirit-lamp or through a platinum tube 
heated to bright redness, the floating matter was sensibly 
destroyed. It was therefore combustible—in other words, 
organic matter.* I tried to intercept it by a large respirator of 
cotton-wool tied round the end of the tube tt\ Close pres¬ 
sure was necessary to render the wool effective. A plug of the 
wool rammed pretty tightly into the tube 11 ' was finally .found 
competent to hold back the motes. They appeared from time 
to time afterwards and gave me much trouble; but they 
were invariably traced to some defect in the purifying-appa¬ 
ratus—to some crack or flaw in the sealing-wax used to ren¬ 
der the tubes air-tight. Without due care, moreover, liquid 
particles may also be carried mechanically over. To prevent 
the entrance of such into the experimental tube, the narrow 
conduit which connects it with the flask F is plugged with 
clean asbestos. Thus through proper care, but not without a 

* Mr. Dancer has recently examined microscopically the dust of Manchester, and 
found it to consist almost wholly of organic particles. 


LTOX GASEOUS MATTER. 


341 


great deal of searching out of disturbances, the experimental 
tube, even when filled with pure air or vapour, contains nothing 
competent to scatter the light. The space within it has the 
aspect of an absolute vacuum. 

An experimental tube in this condition I call optically empty. 

Here follows one of the numerous experiments illustrative 
of this subject. A platinum tube 9 inches long, '0*4 of an 
inch wide, and having within it a roll of platinum gauze, was 
placed in a gas-furnace where it could be intensely heated. 
One end of this tube was connected with the entry stopcock of 
the experimental tube S S / , fig. 31; the other end was open to 
the air of the laboratory. The air was sent first through the 
platinum tube cold, then through the same tube heated to 
various degrees of redness, into the experimental tube, where 
it was subjected to the scrutiny of the concentrated electric 
beam. Here are the results :— 


Quantity of Air 

State of Platinum Tube 

State of Experimental Tube 

15 

in. of 

mercury. 

Cold. 

Full of floating particles. 

15 

99 


Red-hot. 

Optically empty. 

15 

99 


Cold. 

Full of floating particles. 

15 

9 9 


Red-hot. 

Optically empty. 

15 

99 


Cold. 

Full of particles. 

15 

99 


Dull red. 

Optically empty. 

15 

99 


Intensely heated. 

Optically empty. 

30 

99 


Intensely heated. 

Optically empty. 

15 

99 

(admitted quickly). „ 

A perfectly polarized blue 
cloud. 

15 

99 

(quickly). 

Barely visible red¬ 
ness. 

Particles. 

15 

99 

(quickly). 

Intensely heated. 

Blue cloud. 

15 

99 

(slower). 

99 

A very fine blue cloud. 

15 

99 

(very slow). 

99 

Optically empty. 

15 

99 


Cold. 

Full of particles. 

15 

99 

(quickly). 

Red-hot. 

Blue cloud.* 


The polarization of light by such clouds as the blue ones here 
mentioned will receive due attention subsequently. 

A rernarkabty fine experiment may be thus made :—Placing • 
a spirit-lamp underneath the cylindrical beam of the electric 
lamp as it marks its track through the illuminated dust of the 
atmosphere, torrents of what would be infallibly mistaken for 

* In subsequent experiments I found that this * cloud ’ arose in great part from the 
action of the heated air upon the india-rubber joint which connected the platinum tube 
with the experimental tube. 


342 THE ACTION OF RAYS OF HIGII REFRANGIBILITY 

black smoke rise from the flame into the beam. A Bunsen s 
flame produces the same effect. But the action of a red-hot 
poker placed underneath the beam is precisely similar; the 
action of a hydrogen flame, moreover, where smoke is out of the 
question, is not to be distinguished from that of the spirit-lamp 
flame. The apparent smoke rises even when the flame or the 
poker is placed at a good distance below the beam. The action 
is really due to the destruction of the floating matter by contact 
with the heated body. It sends upwards streams of air from 
which everything competent to scatter the light has been re¬ 
moved. This optically pure air, in passing through the beam, 
jostles aside the illuminated particles, the space it occupies 
being black in contrast with the adjacent luminosity. The 
experiment is capable of various instructive modifications, and 
may of course be executed with sunlight. 

It is needless to dwell upon the possible influence of the 
floating organic matter of the air upon health. Its quantity, 
when illuminated by a powerful and strongly concentrated 
beam, sometimes appears enormous. One recoils from the idea 
of placing the mouth at the intensely illuminated focus and 
inhaling the swimming dirt revealed there. Nor is the disgust 
removed by the reflexion that at a distance from the focus, 
though we do not see the dirt, we are breathing precisely the 
same air. The difficulty of wetting it, before referred to, may 
render this suspended matter comparatively harmless to the 
lungs, but when these are sensitive its mere mechanical irritation 
must go for something. Perhaps a respirator of cotton-wool 
might in some cases be found useful.* 


§5. 

Deportment of Nitrite of Amyl. 

I now return to the nitrite of amyl. The action of light 
upon the vapour of this substance is exceedingly prompt and 
energetic. It may be illustrated by simply blowing the vapour 
into a concentrated sunbeam. Or the experiment may be made 
to take the following form :—Connecting the tube 6 of the flask 


* Since this paper was forwarded to the Royal Society these experiments have been 
greatly extended. See Proceedings of the Royal Institution, January 1870; also 
Fragments of Science, 1871-72. 


I 


UPON GASEOUS MATTER. 343 

• 

3? with, the pipe of a bellows, after inflating the latter a sharp 
tap upon its board sends a puff* of vapour through the tube a 
into the air. In a moderately lighted space nothing is seen; 
but when the puff is projected into a concentrated sunbeam, or 
into the beam from the electric lamp, on crossing the limiting 
boundary of light and shade it is instantly precipitated as a 
white ring. The ring has of course the same mechanical cause 
as the smoke-rings puffed from the mouth of a cannon, but it is 
latent until revealed by actinic precipitation.* 

In every one of the numerous experiments made with the 
nitrite of amyl, the chemical energy appeared to exhaust itself 
in the frontal portion of the experimental tube. A dense white 
cloud would fall for a distance of 12 or 15 inches upon the beam, 
while beyond this distance the tube would appear almost empty. 
This absence of action might naturally be ascribed to the 
diffusion of the beam beyond the focus ; but when the light 
was so converged as to bring the focus near the distant end of 
the tube the effect was the same. When, moreover, a concave 
mirror received a parallel beam which had traversed the tube, 
and returned it into the vapour in a high state of luminous con¬ 
centration, the light was ineffectual. The passage of the beam 
through a comparatively small depth of the vapour appeared to 
extract from it those constituents which produced decom¬ 
position. That the vapour was present at the distant end of the 
tube w T as proved by the fact that both with the sun and with 
the electric-light the reversal of the tube instantly brought 
down a heavy cloud. As regards the chemical rays, nitrite of 
amyl is the blackest substance that I have yet encountered. It 
rapidly extinguishes them, leaving behind a beam of sensibly 
undiminished photometric intensity, but powerless as a chemi¬ 
cal agent as far as the nitrite is concerned. 

In these experiments air was employed as the vehicle of the 
nitrite-of-amyl vapour. By varying the quantity sent into the 
experimental tube, it was possible to vary in a remarkable 
manner the character of the resulting decomposition. The most 
splendid diffraction colours could be thus produced, and the 
finest texture could be imparted to the clouds. When pure 
oxygen or pure nitrogen was used, the effect was almost the same 

* By a special arrangement it is easy to obtain such rings 2 inches and more in 
diameter. 


* 


344 THE ACTION OF EATS OF HIGH KEFKANGIBILITY 


as with air. With hydrogen the clonds appeared more delicate 
and lustrous; and they sometimes fell immediately after their 
formation in nebulous festoons to the bottom of the tube. This 
doubtless is to be ascribed to the lightness of the atmosphere 
in which they floated. In many cases, however, the particles 
remained suspended, and some of them continued to float even 
after the tube had been so far exhausted as to produce a 
tolerably good air-pump vacuum. 

An additional effect of considerable beauty and interest was 
obtained in the following way. Permitting the convergent 
beam to play for a time upon the mixture of air and nitrite-of- 
amyl vapour, or, better still, upon a mixture of hydrogen and 
vapour, a coarse cloud is formed. Suspending the action of 
the lamp for a. minute or so, a new distribution of the vapour 
appears to occur; for, on re-igniting the lamp, along its con¬ 
vergent beam, and within the old cloud, a new cloud is precipi¬ 
tated. The tint of this new cloud is a delicate bluish-white, 
and its texture is of exquisite fineness. This precipitation of 
one cloud within another may be obtained a dozen times in 
succession. Or, permitting a parallel beam to pass for a time 
through the coarser cloud, on pushing out the lens so as to 
concentrate the light, the fine cloud comes suddenly down upon 
the beam about its place of greatest concentration. This effect 
also may be obtained several times with the same charge of 
vapour. 

No phenomena of this hind have, I believe, been hitherto 
observed. The necessary conditions for their production are, 
first, that the light should decompose the vapour, and secondly, 
that one or more of the products of decomposition should 
either be a solid, or should possess a boiling-point so high 
. as to ensure its precipitation wdien set free. For though 
chemical action might occur, and be even energetic, if the 
products of decomposition be vaporous and colourless they 
will remain unseen. In the case just considered, the nitrate 
of amyl is in all probability a product of the decomposition 
of the nitrite . The boiling-point of the latter is estimated 
at from 91° to 96° C., that of the former being 149° C. The 
nitrite, therefore, can maintain itself as true vapour in a space 
where the nitrate, at the moment of its liberation, must fall as 
a cloud. 


UPON GASEOUS MATTER. 


345 


§ 6 - 

Iodide of Allyl and Iodide of Isopropyl. 

An exceedingly fine example of actinic action is furnished by 
tlie vapour of the iodide of allyl. The effect of light upon this 
substance was observed on October 7, 1868, but I did not then 
know the meaning of the 4 thin cloud like a kind of smoke ’ 
which showed itself in the experimental tube. On satisfying 
myself regarding the deportment of nitrite of amyl, the iodide 

of allyl occurred to me, and on it experiments were immediately 
made. 

The decomposition of this vapour was slower than that of the 
nitrite of amyl. The slowness,-moreover, augmented rapidly 
as the quantity of vapour was diminished. When only a few 
inches of the mixed air and vapour were in the experimental 
tube the action was very slow. The clouds were formed both 
in oxygen and in air. After the action had been continued for 
some time, the fine purple colour of iodine exhibited itself at 
the end of the tube most distant from the source of lio-ht. 
When hydrogen was the vehicle, the clouds were particularly 
lustrous and beautiful. Here and there also, amid the white 
and coarser sections of a cloud, spaces of delicate blue would 
reveal themselves, reminding one of the colour of a pure sky. 
The words 4 wonderful,’ 4 beautiful,’ 4 lustrous,’ and others of a 
similar nature, occur frequently and naturally in my notes of 
this period; for in those earlier experiments the cloud-forms 
obtained were so amazing, and their colours and textures so 
fine, as to rivet attention upon them alone. 

With long-continued action the colour due to the discharge of 
iodine became very intense. It was strong enough to empurple 
the beam which passed through the air of the laboratory after 
its transmission through the experimental tube, and to colour 
deeply a white screen on which the beam was permitted to fall. 
In what condition was this iodine ? It could be liberated by a 
beam deprived almost wholly of its calorific rays. The tempe¬ 
rature of the experimental tube was indeed so moderate that a 
quantity of iodine placed within it and permitted to saturate 
the space with its vapour, produced a barely perceptible flush 
on a piece of white paper. The far more deeply coloured 


t 

346 THE ACTION OF RAYS OF HIGH REFRANGIBILITY 

iodine revealed by tlie beam in the actinic cloud must, I think, 
have been for the most part liquid, and not vapoious iodine. 

I say liquid, because the substance was probably dissolved by 
the particles of the cloud with which it was so intimately 
mixed. Di-allyl, for example, is a powerful solvent of iodine, 
and it was probably one of the products of decomposition. 

The iodide of isopropyl also capitally illustrates the action of 
light upon vapours. It is more slowly acted upon than either 
the nitrite of amyl or the iodide of allyl; nevertheless, in suffi¬ 
cient quantity, its decomposition is very brisk and energetic. 
Purified air which had bubbled through the liquid iodide was 
conducted into the experimental tube. When the pressure was 
1 inch of mercury, the light playing upon the vapour for five 
minutes produced no action; but when it was 10 inches a blue 
cloud made its appearance in two minutes, and in ten minntes 
it had almost filled the tube. When the pressure was 20 inches, 
the action commenced more quickly, and the cloud generated 
was more dense. The whirling motions of this cloud appeared 
to be more brisk than that of the others examined. With 30 
inches of the mixed air and isopropyl the action began in a 
quarter of a minute, and in five minutes a dense cloud was 
formed throughout the tube. The purple of the discharged 
iodine was also very plain in this cloud. 

§ 7 . 

Deportment of Liquids and of their Vapours towards Bays of 

High Refrangibility . 

In the preliminary notice of these experiments laid before the 
Royal Society in June 1868,* considerable stress is laid upon the 
fact that the same rays are absorbed by the nitrite of amyl in 
the liquid and in the vaporous state. A layer of the liquid not 
more than one-eighth of an inch in thickness was found compe¬ 
tent to withdraw from a powerful bfam nearly all the con¬ 
stituents which could effect the decomposition of its vapour. 

I endeavoured at the time to apply this fact to the solution 
of the question whether the absorption of chemical energy was 
the act of the molecule as a whole, or of its constituent atoms.f 
I tried to show that on the first of these assumptions it is 

f See ‘ Physical Considerations pp. 427 and 428. 


* See page 425. 


UPON GASEOUS MATTER. 


347 


impossible for tlie selfsame rajs to be absorbed bj a liquid and 
its vapour. For absorption depends upon the rate of molecular 
vibration, and reaches its maximum when this rate synchronises 
perfectly with the rate of succession of the sethereal waves. 
Now as the rate of molecular vibration depends upon the elastic 
forces exerted between the molecules, and as it could hardly be 
imagined that these forces would remain undisturbed during 1 
the passage of a vapour to the liquid condition, the fact of the 
liquid nitrite of amyl and its vapour absorbing the same rays 
indicated that the absorption was not molecular. We were 
thus driven to conclude that it was atomic ;* and this conclusion 
was fortified by the consideration already adverted to—that 
were the absorption the act of the molecule as a whole, no 
mechanical ground could be assigned for the falling asunder of 
its atoms. Thus actinic action itself pointed out the seat of 
the absorption. 

A wide, if not entire generality was anticipated for the pro¬ 
position that the same rays are absorbed by a liquid and its 
vapour. When this anticipation was first expressed I believed 
that liquids in general would be found so destructive of the 
effectual rays as to render transmission through moderate 
depths of them sufficient to rob a beam of all power to act 
upon their vapours. This idea, entertained though not ex¬ 
pressed, has not been verified, and the deportment of iodide 
of allyl may be taken as representative of a class of facts which 
contradict it. 

Glass cells were employed varying from one-eighth of an 
inch to an inch in width. Filled with the transparent iodide, 
these cells were placed between the electric lamp and the expe¬ 
rimental tube charged with the iodide vapour. The rays after 
traversing an inch of the liquid produced copious decomposition 
in the tube. A marked distinction was thus proved to exist 
between the liquid iodide of allyl and the liquid nitrite of amyl. 

But the same distinction extends to their vapours. The ex- 

* When I use the word ‘ atomic ’ in contrast with ‘ molecular,’ I by no means pledge 
myself to an absolute limit of divisibility. The molecule may resemble a house, the 
atoms the hard bricks composing that house. But. while it is both convenient and 
correct to regard the house as constituted of bricks definitely bounded, it is by no 
means essential to regard the bricks themselves as absolutely indivisible. The divisi¬ 
bility or non-divisibility of the atoms does not in the least affect the atomic theory aa 
a working conception. 


343 THE ACTION OF RAYS OF HIGH REFRANGIBILITY 


ceeding absorbent avidity of the nitrite-of-amyl vapour, and 
the rapidity with which it deprives a powerful beam of its 
effective constituents, have been already noticed. It is quite 
different with the iodide of allyl. A tube 5 feet long was 
charged with the vapour of this substance, and after it, in the 
same line, was placed another tube 3 feet long charged .with 
the same vapour. On sending a beam through both tubes in 
succession, the 5-foot tube, through which the light first passed, 
was filled immediately with an actinic cloud : but a similar cloud 
was at the same time falling in the second tube. A trans¬ 
mission through 5 feet did not seem to diminish very materially 
the power of the beam. A passage through 1 foot of the 
nitrite of amyl would have been far more destructive. 

As these actions are representative and, I believe, most im¬ 
portant, some recent confirmatory experiments executed with 
these two substances may be here summed up. 

1. The nitrite-of-amyl vapour absorbs with such avidity 
the rays competent to decompose it, that a comparatively small 
depth of the vapour quenches the efficient rajs of a powerful 
beam of solar or electric light. 

2. The iodiae-of-allyl vapour, on the contrary, permits a 
beam to traverse it for long distances without very materially 
diminishing the chemical power of the beam. 

3. The liquid nitrite of amyl, in a stratum one quarter of an 
inch thick, quenches all the rays which could act chemically 
upon its vapour. 

4. The liquid iodide of allyl, on the contrary, in a stratum of 
four times the thickness just mentioned, does not materially 
diminish the power of the beam to act upon its vapour. 

5. A very marked difference exists between the deportment 
of the nitrite of amyl alone, and its deportment when mixed 
with hydrochloric acid. The chemical penetrability of the mix¬ 
ture is far greater than that of the pure vapour. The actinic 
cloud, which with the vapour alone is confined to the anterior 
portion of the experimental tube, extends in the case of the 
mixture through the entire tube. 

6. A beam, moreover, which has been transmitted through 
a quarter of an inch of the liquid nitrite is also competent to 
act chemically upon the mixture, and to produce in it dense 
actinic clouds. 


UPON GASEOUS MATTER. 


349 


Tlie action in tliis last case, though not stopped by tlie liquid 
nitrite, is retarded. Employing first tlie liquid screen, it was 
interesting to observe the sudden development of a fine-grained 
luminous cloud, and its violent tumbling about by the decom¬ 
posing beam, the moment the liquid was withdrawn. The 
action of a solution of the yellow chromate of potash is sub¬ 
stantially the same as that of the liquid nitrite. By the 
successive introduction and removal of a cell containing either 
substance successiv e flashes of actinic energy may be produced 
a dozen times and more in the same vapour. 

The molecular relationship of a liquid and its vapour receives 
new illustration from these experiments. Whatever alters the 
action of the one appears to change in a proportionate degree the 
action of the other. 


§ 8 . 

Influence of a Second Body on the Actinic Process. 

Carbonic acid is decomposed by the solar beams in the leaves 
of plants ; but here it is in presence of a substance, chlorophyll, 
ready, as it were, to take advantage of the loosening of the 
atoms by the solar rays. The present investigation has fur¬ 
nished numerous cases of a similar mode of action. All the 
vapours examined may be more or less powerfully affected in 
their actinic relations by the presence of a second body with 
which they can interact. The presence, for example, of nitric 
acid, or of hydrochloric acid, may either greatly intensify or 
greatly diminish the visible action of the light on many vapours 
which, alone or when mixed with air, are decomposable; while 
the presence of the one or the other of the same acids may pro¬ 
voke energetic action in substances which are wholly inactive 
when left to themselves. 

We need not go beyond the nitrite of amyl for an example of 
this kind. For, prompt and copious as the decomposition of 
this substance is when mixed with air, the energy and brilliancy 
of the action are materially augmented by the presence of hydro¬ 
chloric acid. Let a quantity of the nitrite vapour mixed with 
air be sent into the experimental tube till the mercury column 
sinks, say, 8 inches. Then let the flask containing the liquid 
nitrite be removed and one containing strong hydrochloric 
acid put in its place. Let purified air be carried through 


350 THE ACTION OF RAYS OF HIGH REFRANGIBILITY 

the acid into the experimental tube, until a further depression 
of 8 inches is obtained. On allowing the convergent beam 
to play upon this mixture, a cloud of extraordinary density 
and brilliancy is precipitated. The beam appears to pierce 
like a shining sword the nebulous mass of its own creation, 
tossing the precipitated particles in heaps right and left of it. 
This experiment is very easily made, and nothing could more 
finely or forcibly illustrate the phenomena here under con- 
sideration. 

By varying the proportions of the vapour to the acid we vary 
the effects. For example, the proportion of 1 inch of the nitrite 
vapour to 15 inches of the hydrochloric acid did not pro¬ 
duce so brilliant an effect as the proportion 8 : 8. The same is 
true of the proportion 15 inches of nitrite vapour to 1 inch of 
hydrochloric acid. But in this latter case, though the general 
action was less intense than in the case of 8 : 8, the iridescences 
due to diffraction were much finer. No doubt for each par¬ 
ticular substance a definite proportion exists corresponding to 
the maximum of actinic action.* 

The nitrite of butyl affords another striking example of the 
influence of a second body. With air, or alone, it was not 
visibly affected by the light; there was no cloud formed by its 
exposure. It was also mixed with nitric acid in various pro¬ 
portions, but no visible effect was produced by the beam. 

It was then mixed with air which had been permitted to 
bubble through pure hydrochloric acid, in the following pro¬ 
portions :— 

1. 1 inch of air and vapour to 15 inches of air and acid. 

2. 8 inches „ „ 8 „ „ 

3. 15 inches „ „ 1 inch „ „ 

In the first case a dense and brilliant cloud was immediately 
precipitated. In the second case the precipitation of the fine 
white cloud was confined to the convergent luminous cone, 
coarser particles being scattered through the rest of the tube. 
In the third case the cloud was very coarse and very scanty. 
The experiment indicates that the best effect is obtained when 
a small quantity of the vapour is mixed with a considerable 
quantity of the acid. 


* This might form the subject of an interesting inquiry. 


UPOX GASEOUS MATTER. 


351 


Benzol is also a good example of a substance which, when 
alone, defies the power of the light, but which in the presence 
of other substances is readily decomposed. During the earlier 
stages of this inquiry a vast number of experiments were made 
with benzol and commercial hydrochloric acid. The results well 
illustrate actinic action, but they are not to be accepted as indi¬ 
cative of the action of pure hydrochloric acid. Indeed with the 
pure acid and benzol vapour there is no visible action. 

On the 16tli of November, 1868, 2 inches of air and benzol 
vapour were sent into the experimental tube, and afterwards 
the tube was filled with air which had bubbled through the 
commercial acid. My notes, written at the time, describe the 
action of light upon the mixture as producing a cloud of an 
exquisite sky-blue colour, only more luminous andsethereal than 
the sky. The figure of the cloud was also very wonderful. * 

This cloud was permitted to remain for fifteen hours in the 
experimental tube uninfluenced by light. After this interval it 
was found still floating, being composed of curiously shaped 
granular sections joined together by others of more delicate 
hue and texture. The renewed light set the cloud immediately 
in motion, the granular parts disappeared, and the whole for a 
length of 18 inches resumed its primitive delicate hue and 
texture. At some places it turned to white or whitish-grey, 
but at others it was a pure firmamental blue. It became very 
dense as the light continued to act, and finally developed itself 
into a form of astonishing complexity and beauty. 

The experimental tube had then a current of dry air swept 
through it, and it was afterwards exhausted. Two inches of the 
benzol vapour were admitted as before, and dry air was added 
until the tube was full. It required five minutes’ action of the 
light to develop the faintest visible cloud; even after ten 
minutes’ action the cloud was very faint.* The tube was again 
cleansed and exhausted, 2 inches of the benzol vapour were ad¬ 
mitted, followed by air and hydrochloric acid until the tube was 
full. On starting the light chemical action began almost imme¬ 
diately, and ended by the formation of a cloud throughout the 
tube. The influence of the commercial hydrochloric acid is 
here demonstrated. The interaction of nitric acid and benzol 
will be immediately referred to. 


* It was certainly clue to a residue of the previous charge. 


THE ACTION OF RAYS OF HIGH REFRAXGIBILITY 


Bisulphide of carbon is also an illustration in point. Alone 
or mixed with air it resists the action of the light; in the pre¬ 
sence of hydrochloric or of nitric acid it is responsive to that 
action. On the 17th of November, 1868, for example, the pure 
vapour was admitted into the experimental tube until a depres¬ 
sion of 2 inches of the mercury column was observed. A powerful 
light was permitted to act for twelve minutes upon the vapour, 
but no action was observed. A quantity of air which had passed 
through aqueous hydrochloric acid was then admitted into the 
tube. Six minutes’ subsequent action of the light developed a 
cloud of considerable density. Toluol and other substances 
might here be mentioned in further illustration of this mode 
of decomposition. But I pass over hundreds of these earlier 
experiments which were made chiefly to instruct myself and 
to secure me from error. Some definite results will be given 
further on. 



Generation of Artificial Shies . 


I have now to introduce, though only for partial treatment, a 
subject which might with advantage be kept isolated, but which 
is so mixed up with my notes of 1868 as to be inseparable from 
the descriptions of chemical action which they contain. I refer 
to the blue colour always exhibited at the birth of clouds 
obtained from small quantities of the vapours of active sub¬ 
stances, and often from large quantities in the case of sub¬ 
stances of slow decomposition. The first distinct record of this 
appearance occurs in my notes for October 10, 1868. On the 
9th I had been engaged upon the iodide of allyl with reference 
to its interaction with hydrochloric acid. Small quantities only 
of the vapour had been employed; and it was found that when 
the acid was fresh and strong the action was vigorous, that it 
declined in energy as successive charges of dry air were sent 
through the acid, becoming vanishingly feeble on the fifth filling 
of the experimental tube. 

On the morning of the lOtli the tube used on the preceding 
day was washed with distilled water, and swept out by a current 
of dry air. A mixture of air and hydrochloric acid was then 
sent into it, no vapour of any kind being employed. When the 


UPON GASEOUS MATTER. 


light first passed through it, and for some time afterwards, the 
experimental tube appeared perfectly empty. Slowly and 
gradually, however, upon the condensed beam a cloud was 
formed which passed in colour from the deepest violet , through 
blue, to whiteness . To this record of my note-book the remark 
is added, • connect this blue with the colour of the shy .’ 

In fact it was impossible to avoid Seeing the relationship of 
both. Previous to this entry the blue had attracted my atten¬ 
tion. It was unfailing in its appearance when the action was 
slow. The blue colour was in all cases the herald of the denser 
actinic cloud. I took a pleasure in developing it in connexion 
with general actinic action, and in determining whether, in all 
its bearings and phenomena, the blue light was not identical 
with the light of the sky. This to the most minute detail 
appears to be the case. The incipient actinic clouds are to all 
intents and purposes pieces of artificial shy , and they furnish an 
experimental demonstration of the constitution of the real one. 

Reserving the fuller discussion of the subject for a subsequent 
paper, it may be stated in a general way that all the phenomena 
of polarization observed in the case of skylight are manifested 
by these blue actinic clouds; and that they exhibit additional 
phenomena which it would be neither convenient to pursue, 
nor perhaps possible to detect, in the actual firmament. They 
enable us, for example, to follow the growth and modification 
of the phenomena of polarization from their first appearance in 
the barely visible blue to their final extinction when the cloud 
has become so coarsely granular as ho longer to scatter polar¬ 
ized light. 

These changes, as far as it is now necessary to refer to them, 
may be thus described :— 

1. The incipient cloud, as long as it continues blue, dis¬ 
charges polarized light in all directions, but the direction of 
maximum polarization is at right angles to the illuminating 
beam. 

2. As long as the colour of the cloud remains distinctly blue, 
the light discharged from it normally is perfectly polarized-, this 
light may be utterly quenched by a Mcol’s prism, the cloud 
from which it issues being caused to disappear. Any deviation 
of the line of vision from the normal enables a portion of the 
light to reach the eye in all positions of the prism. 

23 


354 THE ACTIOX OF RAYS OF HIGH KEFRANGIBILITY 

3. The plane of polarization of the perfectly polarized light 
is parallel to the direction of the illuminating beam. Hence a 
plate of tourmaline with its axis parallel to the beam stops the 
light, and with its axis perpendicular to the beam trans¬ 
mits it. 

4. A plate of selenite placed between the Hicol and the 
cloud shows the colours of polarized light, and as long as the 
cloud continues blue these colours are most vivid in the direction 
of the normal. 

5. The particles of the incipient cloud are immeasurably 
small, but they gradually grow in size, and at a certain period 
of their growth cease to discharge perfectly polarized light. 
For some time afterwards the light that reaches the eye, Avhen 
the Nicol is in its position of minimum transmission, is of a 
magnificent blue colour. It is called in the following pages 
the residual blue . 

6. Thus the waves that first feel the influence of size , both at 
the minor and major polarizing limits of the growing particles, 
are the smallest waves of the spectrum. These waves are the 
first to accept polarization and the first to escape from it. 

7. As the actinic cloud grows coarser in texture the direction 
of maximum polarization changes from the normal, enclosing an 
angle more or less acute with the axis of the illuminating beam. 

8. In passing from section to section of the same cloud the 
plane of polarization often undergoes a rotation of 90°- In the 
following pages this is designated as a change from positive to 
negative polarization, or the reverse. 

§ 10 . 

Changes of Polarization in Atinic Clouds. 

The experiments on benzol vapour and hydrochloric acid now 
to be desciibed are of interest on optical rather than on chemi¬ 
cal grounds. They were preceded by other experiments in 
which the vapour was mixed with nitric acid, and a minute re¬ 
sidue of the latter lingering in the experimental tube may have 
influenced the results. The hydrochloric acid employed, more¬ 
over, was the commercial acid, and could not be regarded as 
Pure*. Thus though the decomposition of a vapour was certain, 
that it was not the pure vapour of benzol mixed with pure 
hydrochloric acid gas may be taken for granted. Indeed other 


UPON GASEOUS MATTER. 


355 

experiments executed with the pure acid reduced the action 
to nil. 

Dry air charged with the benzol vapour was permitted to 
enter the tube till a depression of one inch of the mercurial 
column was obtained; half an atmosphere of air charged with 
hydrochloric acid was then added. The action of light on this 
mixture was very powerful. The tube was for a moment 
optically empty, but its transparent contents were immediately 
shaken into a dense and luminous cloud. The normal polari¬ 
zation was feeble, the oblique strong; the selenite colours 
in the former case were weak, in the latter brilliant. When the 
line of vision was transverse, the colours seemed mainly limited 
to red and green. 

The tube was swept with dry air and exhausted. Half an 
inch of air and benzol vapour was admitted, and after it half an 
atmosphere of air and hydrochloric acid. A fine blue colour 
soon appeared, and as long as it continued the direction of 
maximum polarization was along the normal. But a luminous 
white cloud was rapidly generated, the normal polarization 
becoming feeble and the oblique strong. The distant end of 
the cloud, however, continued blue, and in passing from it to 
the white cloud the plane of polarization changed 90°. 

The tube was again exhausted, and a quarter of an inch of 
air and benzol vapour was permitted to enter it, followed by a 
quarter of an atmosphere of air and hydrochloric acid. The 
incipient cloud showed an exceedingly fine blue, the polariza¬ 
tion along the normal being a maximum. The cloud gradually 
thickened at the centre, and finally the polarization there dis¬ 
appeared. As before, when the normal polarization became 
feeble the oblique became strong. 

The tube was once more cleansed and one-tenth of an inch of 
air and vapour was admitted, followed by one-tenth of an 
atmosphere of hydrochloric acid and air. The blue of the 
incipient cloud was here superb, and it lasted longer than in 
the last case. The selenite tints produced by the normally 
polarized light were exceedingly brilliant; but they.faded 
gradually as the cloud passed from blue to whitish-blue. At 
the centre of the cloud the normal polarization first fell to nil 
and then reappeared, having changed, however, from positive 
to negative, the two ends remaining as before. The influence 


356 THE ACTION OF KAYS OF HIGH REFRANGIBILITT 


of attenuation on tlie production of the blue colour is here 
strikingly exemplified. 

The tube containing the benzol vapour was again cleansed 
and exhausted, and the last experiment was repeated ; that is 
to say, one-tenth of an atmosphere of the air and vapour was 
mixed with one-tenth of an atmosphere of hydrochloric acid. 
After ten minutes’ action the actinic cloud was found divided 
into five segments, alternately blue and white. Every two 
adjacent segments of the cloud were oppositely polarized, being 
divided from each other by a section of no polarization. The 
rectangle (fig. 32) represents the several divisions of the cloud; 
the letters B and W denoting the blue and white segments 
respectively. The transverse lines represent the neutral sec¬ 
tions. 

Fig. 32. 


B 

"W 

B 

W 

B 


On the 9th of December, 1868, some experiments were made 
with the nitrite of butyl which merit a passing notice. 

Atmospheric air was permitted to bubble through the nitrite 
until the experimental tube was quite filled with the mixture. 
Fifteen minutes’ exposure produced a very slight action, an 
exceedingly scanty and coarse precipitate being formed. When 
due care is taken the action entirely disappears. 

One inch of the mixed air and vapour was now admitted into 
the experimental tube, and after it half an atmosphere of air 
which had bubbled through aqueous hydrochloric acid. The 
instant the beam passed through the experimental tube an 
intensely white cloud was precipitated. 

The tube being cleansed, one-tenth of an inch of the nitrite 
and air, followed by one-tenth of an atmosphere of air and 
hydrochloric acid, was sent into it. The blue of the incipient 
cloud was in this instance perfectly superb. The polarization 
at right angles to the beam was perfect, and the selenite colours 
exceedingly vivid. As the cloud thickened the polarization 
along the normal disappeared, but it became strong obliquely. 
Two neutral points were observed by oblique vision in the case 
of this cloud. This effect is not uncommon. 

The tube was withdrawn from the light for six minutes ; on 
re-examination the cloud was found to have lost its beauty of 







UPON GASEOUS MATTER. 


357 


form ; and now the cloud-centre, by normal vision, polarized 
the light in a plane opposite to that of the two end3. 

Twelve bubbles of the air and nitrite vapour were then sent 
into the exhausted experimental tube, and after them thirty- 
six bubbles of air and hydrochloric acid; several minutes’ 
exposure produced no action. Three inches of hydrochloric acid 
were then added, and the same superb blue as that noticed in 
the last experiment soon made itself manifest. It faded gra¬ 
dually as the cloud became more dense, and finally merged into 
whiteness. 

The mixture of nitrite of amyl and hydrochloric acid was 
also examined in small quantities; but though the blue was 
fine, it had not the splendid depth and purity of colour 
obtained with the nitrite of butyl. 

§ 11 . 

Early Difficulties and Sources of Error. Action of Infinitesimal 

Quantities of Vapour. 

The whole of the autumn of 1868 was devoted to the inves¬ 
tigation from which I have taken the foregoing brief extracts. 
During this period 100 different substances must, I think, have 
been subjected to examination, and in the case of many of 
them the experimental tube must have been exhausted and 
refilled from 50 to 100 times. In some instances, indeed, the 
largest of these numbers falls considerably short of the truth. 
For a time I had no notion of the delicacy of the inquiry, nor 
of the caution required to prevent the action of infinitesimal 
residues and impurities from being mistaken for the decompo¬ 
sition of substances really inert. The necessity of thoroughly 
cleansing, or renewing, every tube and every stopcock, on pass¬ 
ing from one substance to another, became gradually apparent. 
Water, alcohol, caustic potash, and acids were successively 
employed to cleanse the experimental tubes; but the method 
found most convenient, and that finally adopted, was the 
thorough lathering and sponging-out of the tubes with soft 
soap and hot water, and the flooding of them with pure water 
afterwards. They are then dried with clean towels, and finally, 
polished by passing to and fro within them, by means of a 
ramrod, a clean silk handkerchief. The stopcocks are cleansed 
by suitable brushes ; fresh cocks, a fresh tube, and a fresh plug 
of asbestos being employed for each fresh substance. 


358 THE ACTION OF KAYS OF HIGH REFRANGIBILITY 

From the draft of the present memoir, written in February, 
I take a few notes indicative of the difficulties caused by small 
impurities. Wishing to set my mind at rest with regard to 
nitric acid and hydrochloric acid, I operated for a time upon 
these substances unmixed with any vapour. Fifteen inches of 
air which had been permitted to bubble through aqueous nitric 
acid were sent into the experimental tube. The decomposing 
beam was first sent through a stratum of the liquid acid an inch 
in thickness. It screened the vapour effectually; no visible 
decomposition was produced. In this case, at the beginning of 
the experiment, a few scattered particles were in the tube. 

The cell containing the liquid acid was removed, and a minute 
afterwards a delicate blue colour began to shed itself among 
the floating particles. It augmented in intensity for five 
minutes, but during that time it could be entirely quenched by 
the Nicol, the particles floating in the blue being left intact. 

These floating particles (mechanically carried in) extended 
only about 6 inches down the experimental tube. Beyond 
them was a streak of fine actinic blue perfectly polarized, and 
beyond this again a dusky grey cloud, which showed no trace 
of polarization. 

After ten minutes’ action the cloud had assumed a fair den¬ 
sity, but it suggested doubts whether it was due purely to the 
nitric acid or to the interaction of the acid and some accidental 
impurity. The experiment was repeated four times with sub¬ 
stantially the same results. In all cases the beam when passed 
through the liquid acid proved powerless ; but always on the 
removal of this screen, or on displacing it by a cell of water, 
action was manifested. To all appearance the nitric acid alone 
generated an actinic cloud. 

The experiments, however, did not quite set my mind at rest. 
The tube was cleansed and the stopcocks heated to redness. 
When subsequently exposed the nitric acid required a much 
longer time to develop a cloud. After five minutes’ exposure, 
with no cell interposed, the faintest blue was visible. After 
ten minutes’ exposure the cloud, at first seen with diffi¬ 
culty? was evident for some distance down the tube. By the 
complete removal of residues and by strict attention to the cerate 
employed to make the tubes air-tight, the action thus lessened 
was caused finally to disappear. In each of the experiments 
with nitric acid recorded in the following pages the acid itself 


UP03T GASEOUS MATTER. 359 

was first tried, and not until its perfect visible inertness bad 
been proved was it permitted to mix with the vapour. 

1 also wished to set my mind at rest regarding the action of 
hydrochloric acid. Several experimental tubes were sponged 
with soap and hot water, washed with alcohol, and finally 
flooded with hot water. They were then thoroughly dried and 
mounted. On a first trial most of them showed a feeble actinic 
action, which on a second trial usually disappeared. In one 
case the light generated a fine blue cloud which stretched 
throughout the entire length of an experimental tube 3 feet 
long. One whitish spot only of the cloud discharged imperfectly 
polarized light. The cloud could be utterly quenched by the 
Nicol, with the exception of a small patch of residual blue about 
2 inches long, which was left curiously suspended in the general 
darkness of the tube. 

On thoroughly cleansing with dry air the tube containing 
the cloud, and trying the acid a second time, an exposure of 
twenty minutes was found to produce no action. This and 
many other similar experiments demonstrate the inertness of 
pure hydrochloric acid.* The inert acid of the foregoing experi¬ 
ment was permitted to remain in the experimental tube all 
night. Next morning, when the beam was permitted to play 
upon it, a blue streak became visible in less than a minute. In 
ten minutes the tube was filled with a delicate cloud. This 
was an almost everyday occurrence at the time here referred 
to. There must have been something in the tube in the 
morning which was not there on the preceding night. An 
infinitesimal residue had crept out of the stopcocks, or the 
hydrochloric acid had acted on the cerate employed to render 
the tube air-tight. 

And here I would allude in % passing to an effect which at a 
future stage of this inquiry will be found suggestive of the 
mechanism by which the complex cloud-forms are produced. 
I touched the top and bottom of the experimental tube for a 
moment with my two fingers ; the cloud, which was of exceed¬ 
ing lightness, immediately showed responsive convexion. It 
was wonderfully sensitive to the slightest local change of tem¬ 
perature. Once started in this simple way the motions of the 


* I had previously reported it active: hence these laborious experiments. 


360 THE ACTION OF RAYS OF IIIGII REFRANGIBILITY 


cloud went on, and ended in the development of a splendid 
cloud-figure. 

The influence of a minute residue is also strikingly illustrated 
by the following fact:—Fifteen inches of mixed hydrochloric 
acid and air, exposed for fifteen minutes to a powerful beam, 
showed not the slightest trace of action. A small pellet of 
bibulous paper, not half the size of a pea, was moistened with 
the iodide of allyl. I held the pellet between my fingers till it 
became almost dry, then inserted it into a connecting piece, 
and sent a little air over it into the experimental tube. On 
stopping the flow of air a blue cloud began to form immediately, 
and in five minutes the rich colour had extended quite through 
the experimental tube. This cloud was 3 feet long and dis¬ 
charged a good body of light, but for some minutes it could 
be completely quenched by the Nicol. At the end of fifteen 
minutes a white massive cloud filled the experimental tube. 
Considering the amount of matter concerned in the production 
of this nebula, it seemed like the development of a cloud-world 
out of nothing. 

But this is not all. The pellet of bibulous paper was removed, 
and the experimental tube was cleansed by allowing a current 
of dry air to sweep through it. The current passed through the 
connecting-piece in which the pellet of bibulous paper had rested. 
The supply of air was at length cut off and the experimental 
tube exhausted. Fifteen inches of hydrochloric acid were sent 
into the tube through the same connecting-piece . It is here 
to be noted, 1st, that the whole quantity of iodide of allyl 
absorbed by the pellet was exceedingly small; 2ndly, that I had 
allowed almost the whole of this small quantity to evaporate; 
3rdly, that the pellet had been cast away, and the tube in which it 
had rested had been rendered tfie conduit of a strong current of 
pure air. It was such a residue as could linger after all this in 
the connecting-piece that was carried by the hydrochloric acid 
into the tube, and there acted on by the light. 

A minute after the ignition of the lamp a chemical action 
declared itself by the formation of a faint cloud. It appeared 
first at the focus. In a couple of minutes more a faint blue, 
perfectly polarized along the normal, filled the anterior portion 
of the tube. The blue also extended from the place of most 
vigorous action down the tube. An amorphous cylinder of cloud 


I 


UPON GASEOUS MATTER. 


361 


soon filled the first 10 inches of the tube, and pushed gradually 
down it. It was followed by a complicated cloud-figure, 
and this again by a vase-shaped nebula, fainter than either. 
At the end of fifteen minutes a body of light, which, considering 
the amount of matter involved, was simply astonishing, was 
discharged from the cloud. In one position of the Nicol this 
cloud was a salmon-colour, in the other a blue-green. When 
a plate of tourmaline, with its axis parallel to the beam, was 
passed along in front of the cloud, at some places it showed a 
particularly vivid blue-green. When placed perpendicular at 
these places, the field of the crystal was a yellow-green. 

I doubt whether spectrum analysis itself is competent to deal 
with more minute traces of matter than those revealed by 
actinic decomposition. If the weight of the cloud formed in 
this experiment were multiplied by trillions it would probably 
not amount to a single grain. Bodies placed behind it were 
seen undimmed through the cloud. The flame of a candle 
suffered no sensible diminution of its light. It was easy to read 
through the cloud, a page which the cloud itself illuminated. In 
fact the cloud was *a comet’s tail on a small scale; and it proved 
to demonstration that matter of almost infinite tenuity is com¬ 
petent to shed forth light of similar quality, and in far greater 
quantity than that discharged by the tails of comets.* 

These facts render the statement intelligible that even when 
all reasonable precautions appear to have been taken it is not 
easy to escape every trace of chemical action on first charging 
the experimental tube even with an inert substance. In my 
earlier experiments, when distilled water only was employed to 
cleanse the tube, the first exjDeriment with air alone was sure to 
develop an actinic cloud of a beautiful fern-leaf pattern. And 
even now, after the most careful employment of the soft soap 
and hot water, the first charge of pure nitric, or of pure hydro¬ 
chloric acid often developes an exceedingly delicate blue actinic 
cloud. As regards the optical question, these irregular clouds 
exhibit some of the finest effects. 

One additional fact to illustrate the disturbances incidental to 
this work. Pure nitric acid had been proved over and over again 
to exhibit no visible action; but after its inertness had been 

* The action here referred to has been since developed into a provisional hypothesis 
of cometary phenomena. I shall return to the subject. 


362 THE ACTION OF RAYS OF HIGH REFRANGIBILITY 

demonstrated, a case occurred wliere it pioduced latlier dense 
actinic clouds five times in succession. Indeed tlieie seemed to 
be no end to tlieir possible development. The only thing to which 
this change from inertness to activity could be ascribed, was a 
change in the cerate used to render the ends of the tube air¬ 
tight. On examination it was found that the infinitesimal 
effluvium yielded by the new cerate to the nitric acid was the sole 
cause of the anomaly. Nitric acid, then, produces no actinic 
cloud 5 hydrochloric acid produces no actinic cloud ; air passed 
through potash and sulphuric acid produces no actinic cloud, 
no matter how powerful or how long-continued the action of 
the light may be. 

I had hoped during the present year to be able to go over again 
a vast amount of ground rendered debateable by the discovery of 
such irregular actions as those here recorded. An accident in 
the Alps has unfortunately disqualified me from doing this. But 
as ardent workers have already entered this new field of inquiry, 
I think it right not to postpone the publication of this first 
part of my researches. Not only the descriptions of the de¬ 
portment, but even the names of the vast majority of the sub¬ 
stances with which I have experimented, are omitted. I confine 
myself to eight or ten closely examined and well-established 
cases of actinic decomposition, putting aside for reconsideration 
all such matters as are in the least degree likely to require 
subsequent correction. 


§ 12 . 

Details of Experiments. 

The vapours of the substances mentioned in this section were 
sent into the tube in the manner described in § 3. They were 
mixed, in the proportions stated, with air which had been per¬ 
mitted to bubble through aqueous nitric acid, and the effect 
produced by exposure to the condensed beam of the electric 
lamp is in each case described. 

Toluol (C 7 H 8 ) :— A transparent colourless liquid . 

Contents of Experimental Tube. 

I. Air with toluol vapour . . .1 inch; then 

Air with aqueous nitric acid . .15 inches. 


UPON GASEOUS MATTER. 


363 


On igniting tlie lamp the experimental tube was optically 
empty. 

After thirty seconds the track of the beam through the ex¬ 
perimental tube became blue ; the blue was about as pure as 
that of an ordinary cloudless sky in England. After two 
minutes the colour began to change to a whitish-blue. 

The light discharged normally by the blue cloud continued to 
be perfectly polarized for four minutes after the first appearance 
of the cloud. A rich residual blue was afterwards observed 
when the Nicol was in its position of minimum transmission. 

At the end of ten minutes the residual colour was no longer 
blue, but bluisli-white. Hence the light which first exhibited 
perfect polarization, and which first escaped from perfect polari¬ 
zation, was blue. 

At the end of fifteen minutes a very beautiful cloud-figure 
was developed. The denser portions of the cloud were very 
luminous. 


. 8 inches; then 

. 8 inches. 


II. Air and toluol vapour 

Air and aqueous nitric acid 


The experimental tube was optically empty for a moment at 
starting, but the action was so rapid that in two or three 
seconds the tube was filled with a heavy cloud. At the begin¬ 
ning the colour of the cloud was blue. The incipient cloud, 
which whirled round the beam, discharged for two or three 
seconds perfectly polarized light, but the perfection ceased 
almost immediately. 

The cloud for a time was divided from beginning to end into 
two longitudinal lobes, separated from each other by an appa¬ 
rently empty space about a quarter of an inch wide. When the 
cloud was looked at obliquely in a vertical plane, one of these 
lobes was found to polarize the light positively, the other nega¬ 
tively. In passing from the one to the other the selenite tints 
were reversed. 

The quantity of light scattered by this cloud was very con¬ 
siderable ; it brightly illuminated the walls and ceiling of the 
laboratory. As the cloud became denser, the central empty 
space, which at first divided it into two lobes, gradually dis¬ 
appeared. 

Looked at normally the polarization of the one-half of this 


364 THE ACTION OF EAYS OF HIGH EEFEAXGIBILITY 


cloud was positive, and that of the other negative. Between 
the two a neutral section existed. The oblique polarization of 
the dense cloud was strong. 

III. Air and aqueous nitric acid . . 1 inch; then 

Air and toluol vapour . . .15 inches. 

The action here was not so prompt as in the last case, nor 
was the cloud generated so dense. The cloud-particles, more¬ 
over, were coarser, and showed iridescent colours. Still the 
chemical action w T as distinct and copious. 

Looked at normally, a portion of the scattered light was 
salmon-coloured. The selenite bands appeared to be of this 
colour, and its complementary greenish tint. 

Bisulphide or Carbox (CS 2 ): — A transparent colourless liquid. 

Contents of Experimental Tube. 

. I. Air and bisulphide-of-carbon vapour . 1 inch; then 

Air and aqueous nitric acid . .15 inches. 

On starting the experimental tube was optically empty ; but 
in a minute afterwards the track of the beam became blue, which 
was particularly deep and rich in the middle portion of the 
tube. 

The blue light discharged normally was perfectly polarized, 
but the least deviation of the line of vision from the normal 
caused a portion of the light to pass through the hfrcol. 

The growth of this cloud and the gradual brightening, and 
subsequent whitening, of the blue were very instructive. 

The light discharged normally remained perfectly polarized • 
for seven minutes after the first appearance of the blue colour. 

A faint but rich residual blue was seen for some time after¬ 
wards. 

The selenite colours were exceedingly vivid with this cloud. 
When, moreover, a plate of tourmaline was placed with the 
crystallographic axes parallel to the beam, it was black; placed 
at right angles to the beam, a large portion of the light of the 
cloud was transmitted. 

After ten minutes’ exposure the cloud itself still showed a 
distinct trace of blue. The residual blue was then particularly 
rich and pure. After fifteen minutes the selenite colours were 
still vivid, though the cloud had then become greyish-white. 


UPON GASEOUS MATTER. 


365 


II. Air and bisulphide-of-carbon vapour . 8 inches; then 

Air and aqueous nitric acid . . 8 inches. 

When the lamp was ignited the experimental tube was found 
optically empty; but the chemical action commenced three- 
quarters of a minute afterwards, the convergent beam assuming 
the appearance of a fine blue spear. The action was more ener¬ 
getic than in the last case, though the battery was sensibly 
sinking in power. 

The light discharged normally remained perfectly polarized 
for two minutes after its first appearance. The selenite colours 
were rich and vivid, and the tourmaline in its two characteristic 
positions showed the same striking contrast observed in the last 
experiment. 

In five or six minutes the entire tube was filled with cloud, 
the residual blue being then perfectly gorgeous. 

III. Air and aqueous nitric acid . . 1 inch ; then 

Air and bisulphide-of-carbon vapour 15 inches. 

The tube was optically empty when the lamp was ignited. 
The chemical action soon commenced, a series of layers of blue 
cloud stretching through the entire tube. The action was less 
energetic than in the former cases, this being due in part to the 
sinking of the battery. The light discharged normally remained 
perfectly polarized for ten minutes. 

Cyaxide of Ethyl (C 2 U 5 Cn) :— A transparent colourless liquid . 

Contents of Experimental Tube. 

I. Air and cyanide-of-ethyl vapour. . 1 inch; then 

Air and aqueous nitric acid . .15 inches. 

The tube was optically empty when the lamp was ignited. In 
a minute and a half the track of the beam became distinctly 
blue. The blue light was at first perfectly polarized. 

The beam was crossed by a series of disks, which were 
denser and more whitish than the general mass of the cloud. 
The extinction of these disks by the Nicol was curious and 
interesting. 

The growth of the particles in this case was so slow that the 
light emitted normally continued perfectly polarized for thirteen 
minutes after the first appearance of the cloud. A faint residual 
blue was afterwards developed. 


366 THE ACTION OF EATS OF HIGH REFRANGIBILITY 

II. Air and eyanide-of-ethyl vapour . 8 inches ; then 

Air and aqueous nitric acid . • 8 inches. 

The experimental tube was optically empty for two seconds 
after the starting of the lamp; a fine blue colour was then 
observed upon the upper boundary of the convergent beam. 
The light emitted normally did not remain perfectly polarized 
for more than half a minute. In two minutes the tube was filled 
with cloud, the anterior portion being white, and the posterior 
portion bluish. The posterior portion could be utterly extin¬ 
guished by the Nicol long after the anterior portion had begun 
to show a residual blue. Passing with the Nicol from the 
densest to the least dense portion of the cloud, the residual 
colour changed from a bright blue through a gorgeous Alpine 
skyblue to absolute extinction. 

Looked at obliquely in a vertical plane, the two semicylinders 
into w r hich the cloud was longitudinally divided were found in 
opposite states of polarization. 

This was a truly splendid action. The chemical effect was 
exceedingly vigorous, and the cloud-form fine. 

III. Air and aqxieous nitric acid .' . 1 inch ; then 

Air and cyanide-of-ethyl vapour . 15 inches. 

On starting the light the experimental tube was found opti¬ 
cally empty. In a quarter of a minute, however, the track of the 
beam, which previously had been invisible, was coloured blue. 
The chemical action appeared to exert itself with almost the same 
intensity throughout the entire length of the experimental tube. 

For a brief interval the wdiole of the light emitted normally 
was polarized. Then for a time about tliree-fourtlis of the 
length of the cloud could be quenched by the Nicol, the re¬ 
mainder showing a fine residual blue. This sank from a 
brilliant azure at the densest portion of the cloud through deep 
rich blue to entire extinction. 

The selenite bands were exceedingly vivid long after this 
cloud had ceased to be blue. An immense quantity of polarized 
light was discharged normally, even after the cloud had become 
white. Placed between the cloud and the eye, a plate of tour¬ 
maline with its axis parallel to the beam was practically black, 
while when placed across the beam a bright green light was 
copiously transmitted. 


UPOX GASEOUS MATTER. 


3G7 


In one position of the Nicol this cloud was yellow, in the 
rectangular position it was blue. Here also the chemical action 
was very vigorous, and the cloud-form very fine. 

Benzol (C 6 H 6 ): — A transparent colourless liquid. 

Contents of Experimental Tube. 

I. Air and benzol vapour ... 1 inch; then 

Air and aqueous nitric acid . .15 inches. 

Nitric acid is known to form with benzol nitro-benzol, a 
liquid possessing a high boiling-point. But though the mixed 
vapours were allowed to remain together for ten minutes before 
starting the lamp, when the beam passed through the experi¬ 
mental tube it was optically empty. 

Chemical action commenced a quarter of a minute after the 
ignition of the lamp ; a very delicate blue light was then dis¬ 
charged from the beam, the centre of which was particularly 
bright and transparent. The light emitted normally remained 
perfectly polarized for one minute. 

I looked through the Nicol towards the cloud. For a minute 
it was absolutely extinguished. Continuing 4 to look in the 
same direction the residual colour appeared, and passed from 
a rich deep violet to a hard wliitish-blue. It was exceedingly 
interesting to watch the growth and change of the residual 
colour. At a certain period of its existence it rivalled the 
richest blue of the spectrum. 

In two or three minutes the anterior portion of the tube was 
filled by a thick cloud generated by the beam. The cloud 
rapidly diminished in density as the more distant end of the 
tube was approached. It was composed of two longitudinal 
lobes, which, looked at obliquely in a vertical plane, discharged 
light polarized in planes at right angles to each other. 

When the cloud was looked at normally, the line of vision 
being horizontal, on one side of the centre the polarization was 
positive, on the other side negative. Moved to and fro across 
the neutral section, the sudden expansion and contraction of 
the selenite bands was very curious. 

After twenty minutes’ action the neutral section was 
abolished, and the normal polarization (now feeble) became the 
same throughout the entire length of the cloud. 


368 THE ACTIOX OF EATS OF HIGH EEFEAXGIBILITY 


II. Air and benzol vapour . . . 8 inches; then 

Air and aqueous nitric acid . . 8 inches. 

On sending tlie light through it, the tube was not optically 
empty, but crowded with particles. Through them the beam 
appeared to force its way like a spear, bringing down upon 
itself a finer cloud, which soon swathed and masked the coarser 
spherules. 

This experiment was many times repeated, but it was found 
impossible to bring the benzol and nitric acid together in the 
quantities here employed without the formation of a crowd 
(cloud would hardly be the word) of coarse particles. Chemical 
action had manifestly set in without tlie intervention of the 
light. 

I then varied the quantity of benzol vapour and nitric acid. 
When 2 inches of each were admitted into the experimental 
tube, no particles were seen when the lamp was ignited. A 
quarter of a minute after the starting of the lamp the track 
of the beam became blue. This light remained perfectly 
polarized for a minute. In three minutes a dense cloud had 
filled the tube. In the two rectangular positions of the Nicol 
the cloud exhibited a salmon-colour and a hard bluish-greenish- 
white. 

When the quantities of the two vapours were 4 inches each, 
there were no particles in the tube when the lamp was ignited. 
No doubt the substances were ready to attack each other, 
and in less than a quarter of a minute the beam precipitated 
the attack. The action was exceedingly vigorous. For a 
moment, and only for a moment, the polarization was perfect. 
In less than a minute the rapid thickening of the cloud and the 
quick growth of its particles abolished almost all traces of 
polarization. 

When the quantities were 5 inches to 5, particles were found 
in the experimental tube on starting; and the same occurred 
with all greater quantities. When, for example, the quantities 
were 6 inches to 6, 10 inches to 10, or 15 inches to 15, there 
were invariably particles. In some of the experiments it 
seemed as if the chemical attractions were satisfied before the 
light began, the subsequent action being very feeble. In other 
instances this did not seem to be the case; for though the 
particles existed, the spaces between them became immediately 


UPON GASEOUS MATTER. 


369 


filled by a fine dense cloud when tlie beam passed among them. 
In some instances the precipitation was exceedingly sudden and 
copious. Mr. Cottrell, who has assisted me with zealous intelli¬ 
gence in these experiments, thus describes one result:— e Some 
coarse particles were in the tube on commencing, and these, 
when the light was started, remained perfectly tranquil for a 
moment; but after an instant’s pause the beam appeared to 
pierce like a ploughshare the cloud it had formed, throwing 
right and left of it heaps of precipitated particles. This cloud 
filled the tube almost instantaneously.’ 

To give the benzol and nitric acid more time to act upon 
each other, on Tuesday evening, the 16tli of February, 2 inches 
of each were admitted into the experimental tube, and allowed 
to remain there through the night. Sixteen hours subsequently 
the beam was permitted to act upon the mixture. The tube 
which contained it was to all appearance absolutely empty; no 
particles whatever had formed during the night. In a quarter 
of a minute after starting the lamp chemical action began, 
and in five minutes the beam had filled the tube with a dense 
cloud. 

The deportment of benzol may be thus summed up :— 


Benzol. 


Nitric acid. 


2 

inches. 2 inches. 

No particles; strong actinic action. 

4 


4 

yy 

No particles; very strong actinic action. 

5 

5* 

5 

yy 

Particles; dense actinic cloud precipitated 
among them. 

6 

» 

6 

yy 

„ sometimes ,, ,, 

10 

ff 

10 

yy 

„ sometimes „ „ 

15 

yy 

15 

yy 

„ sometimes „ ,, 

1 

yy 

15 

yy 

No particles; strong actinic action. 

15 

yy 

1 

yy 

Particles. 

Iodide 

OF 

Allyl (C 3 

H 5 I) :— A transparent yellowish liquid. 


Contents of Experimental Tube. 


I. Air and iodide-of-allyl vapour . . 1 inch; then 

Air and nitric acid .... 15 inches. 


The beam traversed the tube for an instant as if the space 
within it were a vacuum, but in the fraction of a second a 
brilliant shower of particles fell upon the beam. The cloud 
became coarse immediately. The action occurred in the an¬ 
terior part of the tube, the most distant part being apparently 
24 


370 THE ACTION OF RAYS OF HIGH REFRANGIBILITY 


free from action. This is quite different from the deportment 
of iodide of allyl and hydrochloric acid. On reversing the tube 
another cloud, of finer texture than the first, was precipitated. 
The cloud assumed beautiful and curious forms. The inner 
portions of its two longitudinal lobes were shaped like screws; 
they moreover rotated like screws, moving as if they were 
pushed mechanically into the mass of cloud in front of them. 
The whole effect was very fine, and the action extremely vigor¬ 
ous. As might be expected from the density of the cloud, the 
normal polarization was almost nil. 

* 

II. Air and iodicle-of-allyl vapour . . 8 inches; then 

Air and nitric acid . . . .8. inches. 

The tube was optically empty at first, but the action, though 
not so brilliant as in the last case, was very prompt and ener¬ 
getic. A«very coarse cloud was rapidly formed throughout the 
entire tube, upon the bottom of which the particles appeared to 
fall in showers. 

The cloud having apparently ceased to thicken, the action of 
the lamp was suspended. On its accidental re-ignition a fine 
cloud, dense and luminous, was suddenly precipitated among 
the coarser particles. On again suspending the lamp the finer 
cloud vanished, but the coarser particles remained. On re-igni¬ 
tion the fine white cloud was precipitated as before, entirely 
masking the coarser one by its superior density and closeness of 
texture. This action was repeated several times in succession. 

Allowing the parallel beam from the lamp to act for a time 
upon the cloud, on changing it to a convergent one the superior 
intensity of the light immediately caused a fine, dense, and 
luminous precipitation. By rendering the beam alternately 
parallel and convei gent, this action could be reproduced several 
times in succession. 

HI. Air and nitric acid .... 1 inch; then 

Air and iodide-of-allyl vapour . 15 inches. 

Immediately after igniting the lamp the action commenced, 
and spread through the entire tube in less than two minutes. 

The falling of the particles in vertical showers occurred here 
also. 

After a time the lamp was extinguished, - and the tube 
was permitted to remain quiescent for an hour. On re- 


UPON GASEOUS MATTER. 


071 
o I 1 

igniting the lamp the tube appeared to be quite empty. The 
cloud that had previously filled it had entirely disappeared. 
Half a minute’s action of the beam brought down upon it 
copious precipitation, a revival of the action occurring after¬ 
wards throughout the entire tube. 

Iodide of Isopropyl CH(CH 3 ) 2 I. 

Contents of Experimental Tube. 

I. Air and iodide-of-isopropyl vapour . 1 inch ; then 

Air and nitric acid .... 15 inches. 

After a moment of apparent emptiness a very splendid action 
set in. A cloud orf exceeding brightness suddenly filled the 
space occupied by the convergent beam. The light scattered 
by this anterior cloud was very powerful. At the distant end of 
the tube the action was feeble. I reversed the tube, but the 
precipitation here was by no means so prompt and copious as 
at the other end, into which the vapour had been evidently 
swept by the air and nitric acid. 

The lamp was suspended for about five minutes; on re-ig- 
niting it a coarse cloud was found within the tube; but instantly 
through this coarseness a finer cloud of exquisite colour, lumi¬ 
nousness, and texture was shed. A violent whirling motion 
was set up at the same time. The longitudinal lobes in this 
case were very curiously found. 

II. Air and iodide-of-isopropyl vapour . . 8 inches; then 

Air and nitric acid . . • . .8 inches. 

Tube optically empty, but in the fraction of a minute a shower 
of very coarse particles had fallen upon the beam. They aug¬ 
mented up to a certain point and then appeared to diminish. 
The reversal of the tube caused fresh precipitation. The ren¬ 
dering of the beam more convergent also caused augmented 
precipitation, but nothing so fine as in the last experiment. 
The action, indeed, was altogether inferior to the last in beauty 
and energy. 

The action of the lamp being suspended for a few minutes ; on 
re-igniting it the tube appeared empty, but in a moment a cloud 
much finer than that at first obtained was precipitated on the 
beam. Curious masses of particles gushed at irregular intervals 


372 THE ACTION OF RAYS OF HIGH REFRANGIBILIT1 


upon the beam On reversing the tube the action was decidedly 
finer than at first. 

Thus, extinguishing the lamp after it has been acting for a 
time, the vapour during the period of suspension undergoes a 
change which enables it to fall as a finer and more visibly 
copious cloud than at the beginning of the action. 

/ 

III. Air and nitric acid .... 1 inch ; then 

Air and iodide-of-isopropyl vapour . 15 inches. 

• 

The action commenced immediately, and in less than a minute 
the beam had filled the tube with an unbroken cloud. The beam 
was rendered parallel, and the action permitted to continue for 
eight minutes. The end nearest the light became rapidly empty, 
while in the distant half of the tube the particles fell in heavy 
showers. The whole tube subsequently became almost empty; 
the disappearance of the dense cloud first generated was very 
striking. It would appear as if after the first sudden precipi¬ 
tation evaporation had set in and restored the particles to the 
gaseous condition. 

t • 

Nitrite of Amyl (C 6 H u ONO):— A transparent yellowish liquid. 

# 

Contents of Experimental Tube. 

I. Air and nitrite-of-amyl vapour . . 1 inch; then 

Air and nitric acid . , . .15 inches. 

The tube was optically empty at starting; the action began 
in half a minute, the cloud particles formed being very coarse. 
In four minutes the anterior two-thirds of the tube were filled 
with a very coarse cloud, the remaining third with a finer one. 
The whole rotated round a longitudinal axis, and the finer 
portion was rolled into a curious spiral form, and was tinged 
throughout with iridescent colours. The normal polarization 
was almost nil, except in the finer part of the cloud, which w r as 
slightly blue. 

II. Air and nitrite-of-amyl vapour. . 8 inches; then 

Air and nitric acid .... 8 inches. 

The tube was optically empty for an instant only, a dense 
precipitation occurring immediately upon the concentrated 
beam. The distant part of the tube, however, was but scantily 


UPON GASEOUS MATTER. 


373 


filled, showing the sifting action of the nitrite vapour. On 
reversing the tube copious precipitation occurred. After ten 
minutes’ exposure the particles tended to settle at the bottom 
of the tube. 

III. Air and nitric acid .... 1 inch ; then 

Air and nitrite-of-amyl vapour . 15 inches. 

The tube was optically empty only for an instant; as in the last 
experiment, a dense cloud was immediately precipitated on the 
cone of rays. Here also the distant end of the tube was pro¬ 
tected by the vapour in front. 

In all these cases the action was distinctly less energetic 
than when the nitrite vapour, mixed with air alone, was exposed 
to the light; and very much less energetic than when hydro¬ 
chloric acid was mixed with the vapour. 

Nitrite of Butyl (0 4 H 9 ONO) : — A transparent yellowish liquid. 

This substance gives no sensible action with nitric acid; but 
with hydrochloric, as already mentioned, the action is vigorous 
and brilliant. Here are a few of the results :— 

Contents of Experimental Tube. 

I. Air and nitrite-of-butyl vapour . . 1 inch ; then 

Air and hydrochloric acid . . .15 inches. 

The action began a quarter of a minute after starting, a very 
white and brilliant cloud forming upon the concentrated beam 
and quickly spreading throughout the tube. 

II. Air and nitrite-of-butyl vapour . 8 inches; then 

Air and hydrochloric acid . . 8 inches. 

The action began in about half a minute, a cloud of com¬ 
paratively fine particles being precipitated in the cone of 
rays, while the distant part of the tube was filled with coarse 
particles. The cloud was coarser, and the action less energetic 
than in the last experiment. 

III. Air and hydrochloric acid . . 1 inch; then 

Air and nitrite-of-butyl vapour . 15 inches. 

After four minutes’ action a number of coarse particles had 
formed in the tube together with a faint scroll of cloud. The 


374 THE ACTION OF RATS OF HIGH REFRANGIBILITY 

action was very feeble. For vigorous action with the nitrite of 
butyl the proportion of the acid to the vapour must be large. 

The hydrochloric acid here emplo} T ed was that ordinarily 
used by chemists in quantitative analysis. The same series of 
experiments was executed with commercial hydrochloric acid, 
and the action found distinctly more energetic than when the 
pure acid was employed. 

Hydride op Caproyl (C 6 H n 0, H) :—A transparent colourless 

liquid . 

Contents of Experimental Tube. 

Air and hydride of caproyl .... 8 inches. 

Air and nitric acid . . . . .8 inches. 

The tube was optically empty at starting. In three quarters 
of a minute a blue cloud had formed. It remained perfectly 
polarized for three minutes; then became gradually white, 
discharging imperfectly polarized light. At the end of ten 
minutes a dense white cloud filled the tube. 

§13. 

Action of Rays of Low Refrangibility . 

For the sake of bringing out the influence of the vibrating 
period, I thought it worth while, to contrast the action of power¬ 
ful foci of dark rays with the feeble foci produced by the conver¬ 
gence of the more refrangible rays of the spectrum. A solution 
of iodine in bisulphide of carbon was employed to hold back 
the luminous part of the electric beam. A cell containing 
ammonia-sulphate of copper was employed to hold back the 
rays of low refrangibility and allow those of high refran¬ 
gibility transmission. The destructive action of the ammonia- 
sulphate on the calorific rays is well known. Its depth in the 
present case was such as to quench completely the red, orange, 
and yellow of the spectrum, but it allowed transmission to the 
violet and blue, and a small portion of the green. The vapours 
employed were mixed with the various acids mentioned in the 
respective cases. 

Nitrite of amyl.8 inches. 

Pure hydrochloric acid.8 inches. 


UPON GASEOUS MATTER. 


O ^ C 

o / 5 

The convergent beam of the lamp was sent through the cell 
containing the solution of iodine, and was permitted to act 
upon the mixed acid and vapour for ten minutes. The am¬ 
monia-sulphate cell was then introduced and the opaque solu¬ 
tion removed. For an instant afterwards the tube was optically 
empty. Then a dense cloud was precipitated, which advanced 
like a moving ploughshare towards the most distant end of the 
tube. Within half a minute after the withdrawal of the opaque 
solution the tube was filled with cloud, which augmented in 
density for five minutes, when the experiment ceased. A repe¬ 
tition of the experiment yielded the same result. 


Iodide of allyl ..8 inches. 

Nitric acid.8 inches. 

Looked at for an instant after the vapour and acid had 
entered, with the white light of the electric lamp, the experi¬ 
mental tube was seen to be optically empty. The opaque 
solution was immediately introduced, and the vapour was sub¬ 
jected to the action of the dark rays for ten minutes. 

The opaque solution was then removed for an instant, and 
the tube was seen to be optically empty. The strong calorific 
rays had produced no action. 

The cell containing the blue liquid was then introduced; in 
less than half a minute the action became visible, and aug¬ 
mented rapidly. In three minutes a cloud stretched quite 
through the tube from end to end. ■ The scattering of the blue 
light by the coarse particles of this cloud produced a very 
pretty effect. 

Benzcfl.4 inches. 

Nitric acid .... ... 4 inches. 

Looked at for an instant after the admission of the vapour 
and acid the tube was optically empty. The opaque solution 
was introduced, and the invisible rays permitted to act for ten 
minutes. The solution was then removed, and the tube was 
examined for a moment with white light. It was optically 
empty. . The blue liquid being interposed, visible action com¬ 
menced minutes afterwards,* and in ten minutes a cloud 
was formed throughout the tube. A repetition of this experi- 

# No doubt it had previously commenced, but it was invisible in the feeble light. 



. A 

376 THE ACTION OF RAYS OF LOW REFRANGIBILITY 

ment confirmed the inaction of the calorific rays, and showed 
the action of the blue rays to be visible a minute after the 
introduction of the ammonia-sulphate cell. 

Toluol.8 inches. 

Nitric acid.8 inches. 

Looked at for an instant after the admission of the vapour 
and acid, the tube was found optically empty. Ten minutes* 
action of the calorific rays produced no effect. The blue liquid 
was then interposed, and in two minutes a cloud was visible 
upon the feeble blue beam. At the end of ten minutes this 
cloud stretched throughout the tube. 

Iodide of /3 propyl..8 inches. 

Nitric acid.8 inches. 

The tube was optically empty at the commencement. At the 
end of ten minutes’ exposure to the calorific rays the tube was 
also empty. The blue cell was introduced, but in two minutes 
after its introduction, no cloud appearing, the cell was removed 
for an instant. The action had begun, though the coarse 
particles of the actinic cloud were too scanty to be seen 
by the weak blue light. The experiment was repeated. As 
before, ten minutes’ action of the calorific rays proved quite 
ineffectual. In one minute after the introduction of the blue 
liquid, no cloud being visible in the tube, the cell was removed. 
A crowd of particles were then seen upon the cone of light. 
The cell was again introduced, and after three minutes again 
withdrawn. The particles had increased considerably. Seven 
minutes’ action rendered them sufficiently numerous to be 
visible in the blue light. After ten minutes the coarse cloud 
was very plainly seen. The action was continued with white 
light after the removal of the blue liquid; it was scarcely more 
energetic than that produced by the blue rays. 

Nitrite of butyl.] inch. 

Hydrochloric acid ...... 15 inches. 

Examined for a moment by white light the tube was optically 
empty. After ten minutes’ exposure to the dark rays the tube 
was again examined by the white beam: it was still optically 
empty. The blue liquid was then introduced, and in a quarter 
of a minute a long streak of cloud had formed. In 2^ minutes 





UPON GASEOUS MATTER. 


377 


a dense cloud was produced which filled the entire tube. An 
exceedingly delicate blue light, and at some parts a deep violet, 
was scattered by this cloud. After five minutes’ exposure to 
the blue ra} r s an intensely white cloud had formed, which com¬ 
pletely filled the tube. The action here was very fine. 

» 

Bisulphide of carbon.8 inches. 

Nitric acid.8 inches. 

The tube was optically empty when the opaque solution was 
introduced; but after ten minutes’ exposure to the calorific 
rays a faint blue tinge was observed, when the opaque solution 
was removed.* The experiment was abandoned, and the mixture 
of vapour and acid was again introduced. At the beginning the 
tube was optically empty; after ten minutes’ exposure to the 
calorific rays it was also empty. In two minutes after the intro¬ 
duction of the blue cell, a cloud became visible: it quickly 
increased, and after four minutes extended throughout the 
tube. After ten minutes’ action a dense wliitish-blue cloud 
filled the entire tube. The experiment was repeated twice with 
the bisulphide, with substantially the same result. 

These experiments are quite conclusive as to the inability of 
the calorific rays to produce actinic clouds : they are the product 
of the more refrangible rays of the spectrum. 

* It is sometimes difficult to get the bisulphide into the tube without this blue 
tinge. It is certainly due to some impurity. With care it can be caused to disappear 
wholly. 


XI. 


AQUEOUS VAPOUR: DISCUSSION RESUMED. 

Analysis of Professor Magnus’s Paper on Gaseous Conduction 

and Absorption. 

1. Gaseous Conductivity. 

The evidence of the action of aqueous vapour on radiant heat adduced in 1864 
and the previous years, was in my estimation so conclusive that I resolved to 
leave the future treatment of the question in the hands of practical meteoro¬ 
logists. But this evidence did not produce the same effect on the mind of 
Professor Magnus, and there is reason to believe that his subsequent papers 
raised in other minds a strong conviction in favour of his views. Inasmuch as 
I have never been able to share this conviction, it behoves me to show the 
reason for my dissent from it, and for my continued reliance on the truth of 
my own results. I therefore propose to resume the discussion, and to subject 
the experiments of Professor Magnus to a more thorough examination than that 
which I have hitherto bestowed upon them. It is a source of deep regret to 
me that he is not amongst us to answer with his own pen the observations and 
arguments now to be adduced. 

In the ‘ Historic Notice ’ of Memoir I. the circumstances under which Pro¬ 
fessor Magnus entered upon the investigation of gaseous conduction and absorp¬ 
tion are given, and in the body and analysis of Memoir II. certain differences 
between his results and mine are referred to. It now devolves on me to give a 
sketch of the origin and character of these differences. It will greatly facilitate 
comprehension if 1 reproduce here the drawings of Professor Magnus’s first 
two instruments, one of which, growing out of the other, yielded the results 
which are at variance with mine. 

One of these instruments was devised to determine the conduction of heat 
by gases, the other to determine the radiation of heat through gases. 

The essential parts of the former are the glass vessel A B, Fig. 1 of the 
annexed Plate, on which is fused the flask C, the top of the one being the 
bottom of the other. The flask C is filled with water, \frhich is kept boiling 
by steam. Its bottom is the source of heat employed in the experiments. 

The receiver A B is immersed in a vessel, P Q, filled with water preserved 
at a constant temperature of 15° C. Into it, and at a distance of 35 milli¬ 
metres (an inch and an half) from the source of heat, is introduced the thermo¬ 
meter f g. Above fg is a screen o o, intended to defend the bulb from the direct 
radiation from the source of heat. Other thermometers are seen carefully 
suspended here and there so as to ensure the constancy of the surrounding tem¬ 
perature. 

Professor Magnus started with the idea that the screen o o, which was usually 
of cork, but sometimes of metal, would completely cut away the radiation from 
the source of heat above it, and at the same time impart no sensible heat to the 



' . 




























































































































































































































































































































































































































7* 











































































































































































































“ 























































ANALYSIS OF PROFESSOR MAGNUS’S PAPER. 


379 


thermometer underneath. He found, however, subsequently that by long- 
continued action (langer dauernder Einwirkung) the screen became warm, and 
that a metal screen was materially less heated than a cork one. With the cork 
screen, however, to use his own words, by far the greater number of the ex¬ 
periments were made. 

The vessel A B being exhausted by an air-pump, and the screen in its place, 
the thermometer/# was permitted to assume its maximum temperature. This 
was found to be 26 7° 0, the temperature of the surrounding medium being 
15° C. The difference 1T7° C., which expresses the rise of temperature in a 
vacuum, is set down by Professor Magnus as 100, and with reference to this 
standard all other temperatures are expressed. 

The vessel A B was then successively filled with atmospheric air and other 
gases, the thermometer, in each case, being allowed to reach its maximum 
temperature. From 20 to 40 minutes were necessary for this. The observed 
temperatures, reduced to the standard before mentioned, are here given:— 


Vacuum . 

100 


Nitrous oxide. 

75-2 

Atmospheric air 

82 


Marsh gas 

76-9 

Oxygen . 

82 


Ammonia 

. 09*2 

Hydrogen 

111-1 


Cyanogen 

. 65-2 

Carbonic acid. 

70 


Sulphurous acid 

. 60*6 

Carbonic oxide 

81-2 




every case but one 

the vacuum- 

-temperature was lowered 

by the introduc- 


tion of the gas. With hydrogen it was augmented, and this Professor Magnus 
considers a conclusive proof that hydrogen conducts heat like the metals. He 
obtains substantially the same result when his vessel is loosely filled with 
cotton-wool or eider down. 

The first question that here arises is: What is this vacuum temperature ? 
How does the heat which raises its temperature 1T7° C. reach the thermometer ? 
Does the vacuum conduct it, and are we to consider the hydrogen a better con¬ 
ductor, and all the other gases worse conductors, than a vacuum P Simple as 
they at first sight appear, it is very difficult to seize upon the exact meaning 
attached by Professor Magnus to his results. The instrument is protected from 
the direct radiation of the primary source, what then is the proximate source of 
heat which warms the thermometer ? It is in a vessel of ‘ very thin * glass, 
surrounded everywhere, save at its upper radiating surface, and the parts 
closely adjacent, by cold water. There cannot, I think, be a doubt that the 
thermometer derives its heat from the screen, which is almost in contact with 
its bulb ; and that the screen, in its turn, is heated by the source of 100° C., less 
than an inch and a half above it. 

Professor Magnus excludes all thought of convection as having anything to 
do w r ith the lowering of the temperature, by the foregoing gases. lie also 
entertains no doubt that every one of them possesses the power of conduction; 
but he concludes that, except in the case of hydrogen, this power is masked 
and overcome by their opacity to radiant heat. ‘ From this,’ he says, ‘ it follows 
that these gases oppose a hindrance to the transmission of radiant heat, and that 
they are athermanous to such a degree that their athermancy exerts a greater 
influence than their capacity to conduct heat.’* 

* I quote from the translation in the Philosophical Magazine. Professor Magnus’s own 
words are, ‘Daraus folgt, dass diese Gase der Yerbreitung der strahlenden Warme ein 
Hinderniss entgegensetzen, und dass sie in solchem Masse atherman sind, dass ihre Aether- 
mansie tinea grossern Einfiuss Ubt als ihre Fahigkeit die Warme zu leiten.’ 





S80 


ANALYSIS OP TROFESSOR MAGNUS’S PAPER 


The first great difference between Professor Magnus’s conclusions and mine 
reveals itself here. In a tube over four feet in actual length, and, because it is 
polished within, much more than four feet in virtual length, I found, in those 
early days, the absorption of radiant heat by atmospheric air and oxygen to be 
0-33 per cent.; whereas, according to Professor Magnus, a stratum of air or oxygen 
an inch and a half in thickness (the distance of his thermometer from his source 
of heat) is able, through its athermancy , to reduce the heat reaching the thermo¬ 
meter from 100 to 82, or to cut of 18 per cent, of the total radiation. 


2. Gaseous Diathermancy. 

The foregoing conclusion from his experiments on conduction led Professor 
Magnus at once to the subject of radiation. The same source of heat was em¬ 
ployed. But here the the vessel A B is mounted on another, F G, Fig. 2, 
resting upon the plate, T T, of an air-pump. S is a thermo-pile with its two 
cylindrical tubes attached to it, the lower one being fixed in a piece of cork resting 
on the air-pump, and the upper one opening towards the source of heat above B. 
Over the thermo-pile is the screen ee, cc, which can be moved aside, or intro¬ 
duced between the pile and the source of heat, at pleasure. The vessel F G is 
immersed in another, M N, filled with water at the constant temperature of 15°C. 

The mode of experiment was this: A B and F G being exhausted by the 
air-pump, the screen ee, cc was moved aside, and the rays of heat passing through 
the tubulure z impinged upon the pile. The consequent deflection w-as noted. 
The gases were then admitted in succession through the cock H, and the 
deflection produced by the radiant heat passing through each of them was 
observed. 

After the removal of the screen, about tico minutes elapsed before the needles 
of the galvanometer took up a fixed position. 

Calling the radiation through the exhausted vessel 100, the radiations through 
the respective gases are given in the following table:— 


Vacuum 

100 

Nitrous oxide 

74-06 

Atmospheric air . 

88*88 

Marsh gas 

72-21 

Oxygen 

88-88 

Cyanogen 

72-21 

Hydrogen 

85-79 

Olefiant gas . 

46-29 

Carbonic acid 

80-23 

Ammonia 

38-88 

Carbonic oxide 

79-01 




In this table there is an absorption of 11-12 per cent, ascribed to atmospheric 
air, and an absorption of 14 21 per cent, to hydrogen. 

Let us compare this result with that already deduced from the experiments 
on conduction. A B is 100 millimetres high, and F G is 175 millimetres; this 
makes the sum of both 335 millimetres. But judging from the drawing, the 
upper face of the pile was about GO millimetres higher than the bottom of F G; 
hence the distance through which the heat had to pas3 to reach the pile was 
about 275 millimetres. In the former experiments the distance between the 
thermometer and source of heat was 35 millimetres. The question therefore 
arises:—If the observed diminution of temperature in both these cases be, as 
alleged, an effect of athermancy, how is it that in the one case a layer of 35 milli¬ 
metres effects an absorption of 18 per cent.; and that in the other case a layer of 
276 millimetres effects an absorption of only 11-12 per cent, of the total radiation? 




OX GASEOUS CONDUCTION AND ABSORPTION. 


381 


3. Proof of Convection. 

Professor Magnus, as alreacty stated, excludes convection from the causes in 
operation; he considers that he has protected himself against this action by 
heating his gases at the . top. In Memoir X. of this series I have had occasion 
to refer to the production of attenuated clouds by the chemical action of light. 
Such clouds reveal in a most instructive manner the motion of the air or other 
gas in which they are suspended. They were contained in glass experimental 
tubes in which the powerful beam of light which produced them also illu¬ 
minated them. Placing the warm finger for a moment on the upper surface of 
the tube, the glass of which might be three, or four, or five millimetres thick, 
the promptness of response on the part of the cloud was extraordinary. It 
would lift itself immediately under the point touched, and turn over right and 
left forming two beautiful cloud spirals, the whole cloud dividing itself at the 
same time into two halves, separated from each other by a transverse black 
septum. In view of this and a thousand previous experiments, I should regard as 
certain that in the vessel A B, heated as it was in Professor Magnus’s experi¬ 
ments, every gas that he examined would establish an ascending current along 
the centre of A B, which would turn over like a mimic trade wind and fall 
along the sides. 

It is easy, however, to transfer the question from the domain of inference to 
that of ocular demonstration. When a little incense smoke, or, still better, a 
small quantity of the fumes of chloride of ammonium, is introduced into the 
vessel A B, crossed by a powerful beam which has been sifted by passing through 
a solution of alum; the currents are seen exactly as, on a priori grounds, we 
should expect them to appear. Here also we have spirals sometimes formed by 
the eddies of convection. The smoke in this experiment must not be uniformly 
diffused, but in streaks or striae. 

Thus observed, the deportment of hydrogen is particularly instructive. In 
this nas the chloride of ammonium rests at the bottom of the vessel until the 
heat is applied above. Then it shoots up in narrow tongues of smoke, which, 
when they reach the top, turn over and fall rocket-like downwards. These 
isolated streamers continued to be formed for a time, but the final action is a 
steady upward stream through the middle of the cylinder, and a descent along 
its sides. With a small glass shade, of the height and diameter of A B, and a 
flask with a concave bottom containing boiling water, this experiment can be 
made with facility. Filling the shade by displacement with hydrogen, and 
blowing into it a little of the chloride of ammonium fumes, it may be allowed 
to stand upon a table. The fumes remain quiescent at the bottom of the shade 
until the hot flask is applied at the top. The currents then begin in the manner 
described. 

I think it certain that to the currents here revealed is to be ascribed the great 
discrepancy existing between Professor Magnus and myself as regards the action 
of atmospheric air, oxygen, and hydrogen upon radiant heat. 

The solution it offers seems in every way complete. Take, for example, the 
experiments on conduction. The warming of the thermometer/// will depend 
upon the rapidity with which the convection currents, falling right and left from 
the hot source, return to the bulb. If during their downward and upward 
journey sufficient time elapse to enable them to yield up their warmth to the 
walls of the vessel, both the thermometer and the source of heat will be chilled 
by their contact. If the convection be rapid, as it is in the case of hydrogen, the 


382 


ANALYSIS OF PROFESSOR MAGNUS’S PAPER 


gas will reach the bulb before it lias wholly parted with its beat. This, I sub¬ 
mit, and not the conductivity of hydrogen, is the cause of the augmentation of 
temperature observed in the case of this gas. 

Take again the experiments on radiation. Between A B and F G we have 
the narrow tubulure s, which checks the propagation of the convection into 
F G. Indeed the smoke in F G may be observed sensibly quiescent, while 
currents are active in A B. In this case the hot hydrogen does not reach the 
face of the pile at all. But the very power of abstracting and transporting heat 
which enabled it to warm a thermometer close to the source of heat operates in 
a precisely opposite fashion when the thermoseopic instrument is at a distance 
from the source of heat. Here the gas takes heat from the source without 
communicating it to the pile. But the consequent diminution of the galva- 
nometric deflection has, I submit, nothing to do with absorption. 

The very ingenious argument founded on the supposed action of hydrogen 
upon radiant heat, and urged by Professor Magnus against me,* must, I think, 
also be abandoned. The contention is that, inasmuch as hydrogen is more 
athermanous than atmospheric air, the greater heating of the thermometer in the 
first experiments could only be due to the greater conductivity of hydrogen.f 

t 

4. Experiments in Glass Tubes ivith Glass Ends. 

Thus far we have dealt with the experiments in which the source of heat was 
a glass surface raised to the temperature of boiling water. Professor Magnus 
subsequently changed his apparatus, and executed a second series of experi¬ 
ments. The recipient of his gases was a glass tube 1 metre long and 35 
millimetres in diameter. Its ends were closed by plates of glass 4 millimetres 
thick. At one end of the tube was placed a strong gas-flame with double 
draught; at the other the thermo-pile. In one set of experiments the tube 
was left naked within, the heat being reflected from its interior surface towards 
the pile; in another set the tube was lined with black paper, which in great 
part abolished the interior reflexion. The arrangement is, in fact, similar to that 
of Dr. Franz already referred to, and it was. accepted by Professor Magnus 
without, misgiving. 

The following numbers express the quantities of heat which reached the pile 
when the two tubes were filled successively with various gases; the radiation 
through the exhausted tubes being set down as 100:— 

Blackened Tube Unblackened Tube 


Vacuum . 


. . 100 

100 

Atmospheric air 


97 56 

85-25 

Oxygen . 


. . 97-56 

85-25 

Hydrogen . 


. . 96-43 

83-77 

Carbonic acid . 


91-81 

78-08 

Carbonic oxide . 


91-85 

72-05 

Nitrous oxide . 


. . 87-85 

75*50 

Marsh gas 


95-87 

76*61 

Olefiant gas 


64-10 

59-96 

Ammonia 


58-12 

55 


* Phil. Mag. vol. xxvi. p. 28. 

t Excluding as he did convection from his thoughts, the reasoning of Professor Magnus 
is strictly logical. ‘ Die Warme,’ he says, ‘ wclche von der untern Flache des Gefasses C 
ausgeht, verbreitet sich in demselben allein durch Strahlung, oder durch StraMung und 
Leitung.’ If this were conceded, his position would be unassailable. When the eider down 
and cotton wool were employed, every filament of them heated by the source must have 
started a convection current. 







ON GASEOUS CONDUCTION AND ABSORPTION. 


n no 
OOJ 

In general the results yielded by the two tubes are widely different from each 
other. Thus in the blackened tube atmospheric air and oxygen absorbed only 
2’44 per cent, of the incident heat, while iu the unblackened tube they absorbed 
14-75 per cent. Hydrogen in the blackened tube absorbs 375 per cent., while 
in the unblackened one it absorbed 1G'27 per cent. These great discrepancies 
are ascribed by Professor Magnus to a change in the quality of the heat by the 
act of reflexion. 

In the memoirs here placed before the reader, these results are ascribed to a 
very different cause; they are in some cases wholly, and in all cases partly, the 
result of convection. With such a source of heat and in such a tube the action 
of properly purified atmospheric air on radiant heat is absolutely incapable of 
measurement by the method described, or even by far more powerful methods. 
This is demonstrated by experiments with glass tubes introduced expressly for 
the purpose of testing both the facts and explanations of Professor Magnus. 
The measurements in the case of the other gases are of course all affected by 
the circumstances here referred to. Take one example, that of ammonia. 
According to the foregoing table, its absorption in the unblackened tube is 
about 3 times that of air; the multiple ought to be raised, at the very least, 
from 3 to 1000. Similar remarks would apply to the other gases. 


The course of the discussion, as far as it is pursued in the preceding Memoirs, 
is indicated in the ‘ analyses but from time to time experiments and reasonings 
not thus touched upon, but which merit a passing reference, were introduced 
on both sides. Of this character is the paper by Professor Magnus referred to in 
the following observations. 


Observations on Professor Magnus’s Paper ‘ On the Influence 
of the Absorption of Heat on the Formation of Dew.’ 

1. Explanatory Remarks. 

In Pog^endorff s ‘ Annalen ’ for 18GG, vol. cxxviii,* Professor Magnus published 
a paper ‘ On the Influence of the Absorption of Ileat on the Formation of Dew.’ 
After speaking of the difficulty of determining the radiation of heat by gaseous 
bodies he goes on to say : * I have, however, made a few determinations of 
the radiation of dry and moist air, and some other gases and vapours. Up to 
the present time (February 18G6), the capacity of these bodies to transmit heat 
has alone been determined.’ For the sake of avoiding misapprehensions which 
he would not countenance, it may be remarked that in my first Memoir 
(January 18G1), I not only deal with transmission, but publish direct experi¬ 
ments on radiation , and show the order of radiation to be the same as that of 
absorption. It is to these experiments that Professor Magnus refers in such 
flattering terms in the letter quoted at page Gl. In my second Memoir, 
moreover ^January 18G2), the dynamic radiation and absorption of both gases 


* Phil. Mag. vol. xxxii. p. Ill 



384 OBSERVATIONS ON PROFESSOR MAGNUS’S PAPER 


and vapours are very amply illustrated, tlie method of heating being so gentle, 
and of such absolute constancy, that the most unstable vapours could bear it 
■without change, and yield accurate results. These are shown to harmonise 
perfectly with others obtained by totally different methods. In my third Memoir 
also (.Tune 1863) elaborate experiments of the same character are recorded, 
and among other forms of the question, vapours and gases dynamically heated 
in one chamber of the experimental tube are caused to radiate through gases 
and vapours enclosed in a second chamber. 

After speaking of former methods and results, Professor Magnus expresses 
himself thus:—‘ Yet there is a complete discordance between the results which 
I had obtained for aqueous vapour by this method, and those which Professor 
Tyndall has obtained by the use of rock-salt plates, and as this physicist, 
although the influence of rock-salt plates is easily ascertained, continually reverts 
to the statement that heat is absorbed by aqueous vapour with several thou¬ 
sand times greater energy than air,* ... I have considered it a duty in¬ 
cumbent on me to compare, if possible, in another manner, the absorption of 
heat by aqueous vapour with that by air.’ 

It is needful for me to explain the words 1 several thousand times’here 
employed. The statement they refer to was made by me in a lecture given 
at the Royal Institution on January 23, 1863, and they -were used to 
render the comparative action of aqueous vapour clear and emphatic to an in¬ 
tellectual, but not wholly scientific audience, f The statement was a perfectly 
definite one. One hundred parts of our atmosphere are described as being 
composed of 99^ parts of oxygen and nitrogen ; and it is stated that the remaining 
0 o per cent, exerts, not several thousand times, but 80 times the action of the 
air in which it is diffused. I then compare the atom of oxygen and nitrogen 
with the molecule of aqueous vapour, and it is the absorptive energy of the 
molecule that is affirmed, and I believe rightly affirmed, to be several thou¬ 
sand times that of the atom. This explanation will probably dissipate much of 
the wonder and the doubt which would rightly attach themselves to the vague 
and unqualified statement ‘that heat is absorbed with several thousand times 
greater energy by aqueous vapour than by air.’ 

Finally, I may be allowed to say one word regarding the slight timbre of 
reproach which many -will detect in the words: ‘ and this physicist, although the 
influence of rock-salt plates is easily ascertained, continually reverts to the state¬ 
ment,’ &c. This, I fear, will be understood to mean that I had persisted in a 
defective mode of experiment when the proofs of its defects were easily attain¬ 
able. I do not think, however, that Professor Magnus "would willingly write 
a sentence that would bear this interpretation. The hygroscopic qualities of 
rock-salt are conceded; care is necessary in the use of it, and especial care 
when air artificially moistened is lloim over it. But as I use it the necessary 
caution is by no means inordinate. I have, for example, permitted my assistant, 
Mr. Cottrell, to experiment for a week at a time with dry and moist air, and to 
detach every evening the rock-salt plates from the experimental tube while the 
latter was filled with moist air. In these experiments no more trace of moisture, 
or its effects, was found upon the plates of salt than could be discerned upon the 

* ‘Da dieser Physiker, obgleich der Einfluss der Steinsalz-Platten leicht zu constatiren 
ist, immer wieder darauf zuruckkommt,’ &c. 

t Under the heading, ‘Radiation tbiough the Earth’s Atmosphere,’ this lecture is 
printed farther on. 


OBSERVATIONS ON PROFESSOR MAGNUS’S PAPER. 


385 


most carefully dried and polished surface of flint-glass or rock-crystal. It 
•will also be borne in mind that in the autumn of 1802 Professor Magnus 
himself examined my plates of salt without being able to detect a symptom 
of that ‘ influence ’ which in the above passage he declares to be ‘ easily 
ascertained.’ 


2. Discussion of Paper. 

In this inquiry Professor Magnus seeks to determine the absorptive power of 
aqueous vapour by experiments on its radiative power. Compared with air, Pro¬ 
fessor Frankland had found this to be high. Professor Magnus now takes up the 
subject, pursuing a method of his own. ‘The gases and vapours were passed 
through a brass tube 15 millimetres in internal diameter, placed horizontally, 
and heated by gas-flames. One end was bent upwards so that the heated air 
ascended vertically, while at a distance of 400 millimetres from the vertical 
current was placed the thermo-electric pile.’ 

When dry air was forced through this tube, the deflection of a delicate 
reflecting galvanometer was 3 divisions of the scale; when air which had pre¬ 
viously passed through water of 15° C. was forced through the tube, the 
deflection rose to 5 divisions. When the water was from 60° to 80° C. in tem¬ 
perature, the deflection was 20 divisions ; w r hile when the water was caused to 
boil violently, the deflection was 100 divisions. But in this last case a mist 
was always visible in the air; the large deflection, therefore, could not be 
considered as caused by radiation from vapour. 

But in the case of the 20-division deflection no mist was visible: still 
Professor Magnus ‘ is tempted to conclude that this deflection also depended on 
the formation of a mist at the boundary of the ascending current.’ 

But the most important—and it is really an important—consideration urged by 
Professor Magnus has reference to the formation of dew. Having described 
various experiments made with the heated tube, he brings forward his main 
argument with much force and clearness. ‘Ido not,’ he says, ‘think that 
these experiments were needed, for if aqueous vapour were really so good an 
absorber of heat as Professor Tyndall maintains, dew could never be formed, 
for the aqueous vapour which is indispensable for dew would at the same time 
form a covering on the surface of the earth and prevent its radiation. But just 
where the atmosphere is particularly rich in water, as in the tropics, dew is 
principally deposited. ... It would then,’ he adds, ‘ be inexplicable that 
clouds could prevent dew.’ 

I may say that I had long previously looked at this difficulty, and in the face 
of it had affirmed the absorptive power of aqueous vapour. For, I reasoned, 
where the vapour is abundant, as, for instance, in the valle} r of the Nile, an 
inconsiderable lowering of temperature will suffice to bring it down ; while if 
the vapour were not abundant, the requisite opening to radiation would be 
offered and the requisite lowering of temperature produced. I have already 
referred to the influence of wet straw in preventing the formation of ice in 
India. But a far more striking demonstration of the influence of aqueous 
vapour on the maintenance of temperature is furnished by the well-reasoned 
observations of Strachey at Madras, which are recorded in the ‘ Philosophical 
Magazine ’ for July 1806. From them 1 will choose a particularly striking 
sample, in wfliich terrestrial radiation was determined at night during a period 

25 


386 OBSERVATIONS ON PROFESSOR MAGNUS’S PAPER. 


when 1 the sky remained remarkably clear, while great variations in the quan¬ 
tity of vapour took place.’ The hrst column of the annexed table gives the 
vapour-pressure, and the second the fall of the thermometer from 6.40 p.m. to 
5.40 a.m. 


Pressure of vapour 
0*888 inch 
0*849 „ 

0*805 „ 

0'749 „ 

0*708 „ 

0*059 „ 

0-605 „ 

0*554 „ 

0*435 , 


Fall of thermometer 
6 * 0 ° 

7-1° 

8*3°' 

8*5° 

10-3° 

12 - 6 ° 

12 - 1 ° 

13*1° 

16-5° 


Now it appears to me that where the supply of vapour is abundant and the 
air nearly saturated, the lowering even of 6° observed in the first observation 
might bring down copious dew. But the table nevertheless forbids the thought 
that aqueous vapour does not exert a great influence on the terrestrial radia¬ 
tion, for the halving of its quantity more than doubles the fall of the thermo¬ 
meter. It is to be observed also that in many places where dew falls heavily, 
the stratum of vapour, though heavy, is shalloio. I have often seen the orange 
trees of Mentone teeming with dew near the level of the sea, while very little 
was to be seen on the hills 100 feet above the sea level. 

General Strachey also showed the influence of aqueous vapour on solar 
radiation. In March 1850, the following augmentations of temperature between 
5.40 a m. and 1.40 p.m. were observed, the corresponding vapour-pressure being 
appended:— 



inch 






Pressure of vapour . 

0*824 

Fahr. 

0-737 

0-670 

0-576 

0-511 

0*394 

Rise of temperature from ) 
5.40 a.m. to 1.40 r.M. \ 

12-4° 

15-1° 

19-3° 

22-2° 

24*3° 

27-0° 


Showing the freer transmission of the solar rays as the vapour diminishes. 

I think there is little doubt that the gaps observed by Sir J. Herschel in the 
ultra-red region of the spectrum, which have been so successfully investigated 
of late by M. Lamansky,* a pupil of Helmholtz, arise from the attack of aqueous 
atmospheric vapour. Indeed M. Lamansky noticed them to become deeper on 
days when the humidity was great. 

In passing, I may remark that the radiation from aqueous vapour which is in 
part the subject of Professor Magnus's paper, may be simply illustrated by 
the hvdrogen flame; its emission, however, being less than one-half that of a 
luminous gas-flame of equal size. Why this feeble emission? Simply, I 
believe, because of the extreme attenuation of a flame of hydrogen. The 
flame is not necessary to exhibit the radiation. I placed an argand burner 
consuming hydrogen in front of a thermo-electric pile, and defending the pile 
from all radiation from the flame itself, allowed the ascending column of 
aqueous vapour to radiate against it. The needle was sent up to its stops at 
90°. At a distance of ten or twelve feet from the column of hot vapour the 
action was very sensible. In causing the vapour to ascend into a powerful 


* 31onatsbericlite for December 1871. 



REMARKS ON THE PAPER OF FROFESSOR MAGNUS. 387 


beam of light, it could be observed, not as a white cloud or mist, which would 
be infallibly revealed by this method of illumination, but by a space as black 
as night , from which the floating matter of the air had been ejected. Neither 
at the edges nor in the midtile was the slightest trace of precipitation. The 
radiation is due to the perfectly invisible mixture of air and vapour. I think 
it quite likely, however, that in Professor Frankland’s experiment mist may 
have been precipitated, or what is still more probable, water in fine particles 
may have been carried mechanically along with the jet of air. 


Appended to the translation of Professor Magnus’s paper, -which was pub¬ 
lished in the ‘ Philosophical Magazine ’ for August 18G6, are the following 
remarks written by me at the time :— 


First Remarks on the Paper of Professor Magnus. 

I should refrain for the present from making any remark upon the paper of my 
friend Professor Magnus, did I not fear that my silence might be misconstructed 
by meteorologists, and that they might be withheld, through a doubt as to their 
value, from prosecuting observations which, I think, are sure to expand the 
boundaries of their science. • 

For an abstract of the experiments and reasonings by which each successive 
objection which has been urged against my experiments on the action of aqueous 
vapour on radiant heat has been met, I would refer to the second edition of my 
work 1 On Heat,’ pp. 381-421. With the desire there manifested to get at 
the bottom of the difference between us, I approach the latest objections of 
Professor Magnus, regretting only that, being on the point of quitting London, 
I can do no more than jot down a few of the more obvious reflexions suggested 
by his own description of his experiments. 

Professor Magnus now infers the absence of absorption from the absence of 
radiation. He employs as source of heat a stream of air, which is first urged 
through water at the ordinary temperature, and afterwards caused to pass 
through a hot brass tube 15 millimetres in diameter. On its emergence from the 
tube it radiates against a thermo-electric pile placed at a distance of 400 milli¬ 
metres. When dry air was urged through the tube, the deflection was exceed¬ 
ingly small; when air moistened as above was employed, the deflection was 
but slightly augmented. 

Now, in the first place, the amount of vapour taken up by air in its passage 
through cold water is so small, and the stream of such air employed by Pro¬ 
fessor Magnus is so thin, that the heat radiated from the vapour must be 
excessively minute. Supposing the vapour compressed only to the density of 
ordinary atmospheric air, the average thickness of the radiating layer would be 
less than ^th of an inch. Even assuming the rays from this source of heat 
to reach the pile without impediment, its action would be inconsiderable, if not 
insensible. 

But the rays were not permitted to reach the pile without impediment. I as¬ 
sume that Professor Magnus did not deem it necessary to png the air intervening 
between his source of heat and his pile j otherwise he would have mentioned a 



388 REMARKS OX THE PAPER OF PROFESSOR MAGNUS. 

precaution of such importance. Here, then, we have the vapour of a cylindrical 
column of air, 15 millimtres thick at its widest part, radiating through a layer 
400 millimetres thick, of a substance intensely opaque to its radiation. Con¬ 
sidering, then, the feebleness ol it3 origin and the difficulties in its way, it is not 
surprising that the radiation from the source of heat chosen by Professor Magnus 
failed to produce any very sensible impression upon his galvanometer.* 

It must be borne in mind that, to obtain copious radiation from a substance 
so attenuated as aqueous vapour, a considerable length of it mast be employed. 
An example will illustrate this. XV hen enclosed in a tube 3 feet long, the 
radiation of sulphuric-ether vapour, at 0-5 of an inch of pressure, exceeds that 
of olefiant gas at the pressure of the atmosphere. In a tube 3 inches long, on 
the contrary, the radiation from the gas is more than treble that from the 
vapour.t That carbonic acid gas excels aqueous vapour in the experiments of 
Professor Magnus, does not therefore surprise me. 

I have at present no means of judging of the validity of the assumption of 
Professor Magnus that the air urged through water at a temperature of 60° or 
80° C., produced its effect by precipitation. There is none visible. By a 
similar assumption, he explains the experiment of Professor Trankland, in which 
aqueous vapour was discharged along the axis of a cylinder of hot air and hot 
carbonic acid, being thus protected by its gaseous envelope. 

'With regard to the formation of dew, the amount deposited depends on the 
quantity cf vapour present in the air ; and where that quantity is great, a small 
lowering of temperature may cause copious precipitation. Supposing 50, or 
even 70, per cent, of the terrestrial radiation to be absorbed by the aqueous 
vapour of the air, the uncompensated loss of the remaining 30 would still pro¬ 
duce dew, and produce it copiously where the vapour is abundant. Attenuated 
as aqueous vapour is, it takes a good length of it to effect large absorption. I 
have already risked the opinion that at least 10 per cent, of the earth’s radia¬ 
tion is intercepted within 10 feet of the earth’s surface; but there is nothing 
in this opinion incompatible with the observed formation of dew. A surface 
circumstanced like that of the earth, and capable of sending unabsorbed 80 or 
90 per cent, of its emission to a distance of 10 feet from itself, must of necessity 
become chilled, and must, if the vapour be abundant, produce precipitation. 

, I would now leave to others the further development of this question, feeling 
assured that, once fairly recognised by field meteorologists, the evidence in 
favour of the action of aqueous vapour on solar and terrestrial radiation will 
soon be overwhelming. An exceedingly important instalment of this evidence 
was furnished by Lieut.-Colonel Strachey in the ‘Philosophical Magazine’ for 
June 18G2. It is especially gratifying to me to find my views substantiated by 
so excellent an observer and so philosophical a reasoner. 

Let me say, in conclusion, that nothing less than a conviction based on years 
of varied labour and concentrated attention could induce me to dissent, as I am 
forced to do, from so excellent a worker as Professor Magnus. Hitherto, how¬ 
ever, our differences have only led to the shedding of light upon the subject; 
and as long as this is the result, such dillerences are not to be deprecated. 

Royal Institution, July 2, 1866. 

* We have no means of judging the humidity of the radiating air as compared with that 
of the air through which it radiated. If the water employed to saturate the air were very 
cold, the latter might he the greater of the two. 

j- Section 14, Memoir II. 


PROFESSOR TTILD’S EXPERIMENTS. 


389 


Professor Wild’s Experiments.* 

I intended the foregoing remarks on the paper of Professor Magnus to he my 
last contribution to this discussion, believing that in the hands ot practical 
meteorologists the alleged power of aqueous vapour would soon receive either 
confirmation or refutation. But already a new experimenter had appeared 
in the person of Professor Wild, of the University of Berne. He describes 
with consummate clearness the state of the question ; and especially points out 
the radical difference between Professor Magnus and myself in the experi¬ 
ments on dry and humid air with open tubes. Professor Magnus, it will be 
remembered, obtained results diametrically opposed to mine, observing heat 
where I had observed cold, and cold where I had observed heat, and notwith¬ 
standing the verification of my results by Professor Frankland, there was no 
indication on the part of the Berlin philosopher that he had either corrected his 
experiments or changed his opinion. It was to this point of difference between 
us that Professor Wild addressed himself. 

From three distinct series of experiments, executed with the utmost skill and 
care, Professor Wild drew a conclusion which is best expressed in his own 
words :_‘ In conclusion, I may say that in all my experiments conducted accord¬ 

ing to Tyndall's method, which included more than a hundred distinct observa¬ 
tions, I have never obtained deflections of the galvanometer needle in contradic¬ 
tion to the statements of Professor Tyndall; that, further, my measurements give 
approximately the same ratio of the absorption by moist air to that by coal-gas ; 
and that, lastly, I consider that certain objections which might have been raised 
against the conclusiveness of Professor Tyndall's method have been removed by 
means of appropriate changes in his method of experimenting. 

< This complete verification of Tyndall’s results rendered it more desirable to 
investigate the absorption of dry and moist air by the method of Professor 
Magnus.’ 

He does so, and corroborates all that I had stated regarding dynamic 
heating and chilling. He also corroborates my statements regarding convection. 
But his observations and conclusions are best stated in his own words: 1 The 
observations with the apparatus again showed strong deflections of the magnet- 
mirror of the galvanometer when air was leaving or entering the tube, the 
former case indicating a chilling, and the latter a warming of the upper face of 
the thermo-electric pile. These deflections were much more intense than those 
produced by the total radiation of the upper source of heat of 100° C. against 
the pile.’ This is exactly what I had found them to be. Professor Wild 
could not obtain a stationary temperature, a::d the irregularities were sufficient 
to mask any difference existing between dry and moist air. He makes this 
noteworthy observation: 1 Coal-gas, one ot the most powerlul absorbers, pro¬ 
duced a deflection of only 40 divisions of the scale, whereas the difference of 
action between air at the pressure of the atm osphere and the same air at 1G 
millimetres pressure, amounted to from 100 to 200 divisions ot the scale. lie 
rightly concluded that this enormous difference could not be a difference ot 
absorption, but one of convection.^ When the air which chilled it was removed, 
the source rose to its pristine temperature. 

* Pogg. Ann. vol. cxxix. p. 57 ; Phil. Mag. vol. xxxii. p. US. 

+ Professor Magnus replies to this that M. Wild’s tube, though heated at the top, 
was of metal, and conducted the heat from the source a certain distance downwards; 


390 


PROFESSOR WILD’S EXPERIMENTS. 


He winds up thus: 1 The insufficient sensitiveness on the one hand, and the 
convection currents on the other, caused me at length to abandon the expeii- 
ments by Magnus s method. And although this method of in\estigating 
absorption mav, in the hands of so experienced and expert an experimenter as 
Professor Magnus, be an appropriate one, I feel bound from my own experience 
to give a decided preference to Tyndall’s method, not only on account of the 
greater facility with which it furnishes qualitative [quantitative] results, but 
also because of its greater delicacy. It is principally in consequence of this 
greater delicacy that, notwithstanding the negative results furnished by Magnus’s 
method, I maintain that the greater power of moist air, as compared with 
dry, has been fully established by the experiments made according to Tyndalls 
method; and I am of opinion that meteorologists may without hesitation 
accept this new fact in their endeavours to explain phenomena which had 
hitherto remained more or less enigmatical.’ 


Professor Magnus’s Last Paper.* 


Stimulated by the investigations of Professor Wild, Professor Magnus again 
took up the subject in 1867. He arranged his apparatus after the pattern of the 
Swiss philosopher, and verified with it the experiments of Professor Wild, 
which were substantially those that he had seen during his visit to London in 
1862. 

But he soon observed an effect new to him, and furnishing what he considered 
a complete explanation of the results obtained by Professor Wild and me. 

The thermo-pile, with its two conical reflectors, was placed between two 
open metal tubes; at the ends of the tubes most distant from the pile were 
placed two cubes of boiling water, which radiated through the tubes against 
the opposite faces of the pile. The arrangement, in fact, was the method of 
compensation. By means of a movable screen the radiation from the two sources 
of heat could be caused to neutralize each other, and by means of a bellows moist 
air, or dry, could be forced into the two tubes. Professor Magnus found that after 
the needle had been brought to zero, and when it might be expected that the 
blowing of the same kind of air into both tubes ought to produce no change in 
the position of the needle, a deflection occurred. When the tubes were examined 
the effect was found to depend on the state of the interior surface. 

Acting upon a first hint of this kind, he lightly coated one of his tubes with 

whereas his tube was of glass and very thin. I'have remarked in another place that Pro¬ 
fessor Magnus’s glass vessel must at all events have been thick enough to bear the pressure 
of the atmosphere, for it had been frequently exhausted. There could only have been a 
difference of degree between M. Wild and him. In his account of the apparatus employed 
in his experiments on conduction, Professor Magnus expressly says, ‘The portions of the 
surface adjacent to the vessel of boiling water are heated by conduction.’ It is the self¬ 
same apparatus that he employs in his experiments on radiation, with this difference, that 
the heat must extend further in the latter case, because here his apparatus, instead of being 
surrounded by water, is surrounded by air. See Plate facing page 378. 

* Fogg. Ann. vol. cxxx. p. 207 ; Phil. Mag. vol. xxxiii. p. 413. 


\ 




professor Magnus’s last paper. 391 

lampblack, and left tlie other polished. When moist air was blown into both, 
the deflection of the needle proved the loss of h^at in the polished tube to be 
greater than in the blackened one. In fact, while the one withdrew 14 per 
cent, the other withdrew 3'7o per cent, of the total radiation. 

Coating the tube with a still stronger layer of lampblack, he found the v 
chilling by moist air not only nil, but negative ; that is to say, a heating effect 
was observed. This was also the case when the tube was lined with cotton 
velvet. When a smooth pasteboard lining was inserted into the experimental 
tube neither heating nor chilling was observed, while a second pasteboard tube 
coated with coarse paper produced a heating etlect. It was known that porous 
bodies, such as lampblack and bibulous paper, were heated by the condensation 
of aqueous vapour in their pores, and to an effect of this class Professor Magnus 
rightly ascribed the warming of the interior of his tube by the moist air. 

But in a previous inquiry he had added another fact, which was not so well 
known ; namely, that metals, as well as non-metallic bodies, were also warmed 
when moist air was blown against them, and chilled by a.current of dry air of 
the same temperature. The first action he concludes is due to the formation of 
water and the liberation of latent heat} the second action he ascribes to the 

evaporation of the water thus formed. 

Placing a linear thermo-pile in contact with his experimental tube, Pro¬ 
fessor Magnus found that heat was produced when moist air was forced 
through it, and cold when it was swept by dry air. Hence the following 
explanation of the results which Iliad ascribed to aqueous vapour. By the 
condensing action of the interior surface of the experimental tube, that surface 
covers itself with a liquid layer of the very highest degree of opacity to 
radiant heat; and it is the renter acting on the rays impinging against the sur¬ 
face of the tube tiiat produces the effect ascribed by me to the vapour of 
water. Professor Magnus does not consider this liquid layer to be continuous, 
but broken into parcels, which not only absorb , but scatter the incident heat. 

It is to be borne in mind that the layer of liquid is altogether invisible, and 
indeed eludes the eye when the most powerful means are employed to detect 
it. Still it covers all substances—resinous, vitreous, or metallic—at all times and 
in the driest climates. It exists,.for example, on the glass of the electrician 
after he has taken the utmost pains to warm and dry it with a view to its 
insulation. This is not stated in so many words by Professor Magnus, but 
this and much more than this, is involved in his explanation. 

Professor Magnus adds to those just referred to, another remarkable 
observation. Air at a temperature of 10° or 17° C. was saturated with mois¬ 
ture, forced through a hot tube which raised its temperature to 38° C., and 
then caused to impinge upon a thermo-pile possessing the same temperature 
of 38° When the air was moist it warmed the face of the pile, when diy 
it chilled it. This occurred not only when the face of the pile was coated 
with lampblack, but also after the coating had been removed, and the metal 
ends of the elements exposed to the current of air. Professor Magnus inters 
from this that aqueous moisture is condensed upon a surface 22 C. higher in 

temperature than the dew point of the vapour.* . , , 

This property of condensing vapours on the surface of bodies is named by 

• To render this experiment secure, no moist air must impinge against the space tome- 
diately surrounding the pile, hut it is not said that this space was protected. 





392 


PROFESSOR MAGNUS’S LAST PAPER. 


Professor Magnus vapour-hesion. IT© extended his experiments to alcohol, 
found in its case the vapour-hesion to be far stronger than in the case of aqueous 
vapour, and the amount of heat withdrawn by the liquid layer overspreading 
the interior surface of the experimental tube much greater. 

But in the tube a length of aqueous vapour is always associated with the 
assumed layer of water 5 hence it would be exceedingly desirable to break tlii3 
association and to determine the action of the water stratum, pure and simple. 
By means of concave and plane mirrors, Professor Magnus reflected the rays 
from a source of heat to his pile. Dry and moist air were urged in succession 
against the mirrors, but unless he visibly wetted them no change whatever wa3 
observed in the amount of the reflected heat. 

This is accounted for by saying that the loss of heat in a single passage to 
and fro across the layer, vanishes in comparison with the heat withdrawn by 
the multiple reflexions within the experimental tube. So frequent are these 
reflexions in the case of some of the heat-rays that Professor Magnus pictures 
them as describing a spiral on the surface of the tube. He does not, however, 
consider that the greater distance traversed b^y the heat-rays in the polished 
tubes can have contributed in any sensible degree to the greater loss of heat 
observed in them. The quenching of the heat is the act of the liquid layer, 
and not of augmented distance. 

While denying any sensible action to aqueous vapour, Professor Magnus 
concedes it to alcohol, which he finds competent to absorb heat even when it is 
discharged in the open air. Many years previously this experiment had been 
made, not only with alcohol, but with every vapour that I had subjected to 
examination. It was indeed one of the many checks introduced to protect me 
from error. 


Remarks on Professor Magnus’s Last Paper. 

I should like to make one remark on the historic opening of this paper, which 
runs thus:—‘Mr. Tyndall found that aqueous vapour absorbs heat to so ex¬ 
traordinary an extent that, when a single atom of oxygen and nitrogen is 
compared with a single molecule of aqueous vapour, the latter absorbs 10,000 
times more heat than the former. He subsequently says that humid air in a 
tube 4 feet long absorbs from 4-2 to G per cent, of the radiation.’ It is to be 
borne in mind that the two statements placed here in juxtaposition are in entire 
harmony with each other. The word 1 extraordinary’ has a relative significance, 
and might perhaps be more fitly applied to the transparency of the air, which is 
truly astonishing, than to the opacity of the vapour. In the place from which 
Professor Magnus quotes,* it is observed that, for every molecule of aqueous 
vapour, there are about 200 atoms of oxygen and nitrogen ; and as experiment 
showed the total vapour to exert eighty times the effect of the total air, ifc 
followed that the single molecule exerted 16,000 times the effect of the single 
atom. 


* The article entitled ‘ Radiation through the Earth’s Atmosphere,’ p. 422. 



REMARKS ON PROFESSOR MAGNUS’S LAST PAPER. 393 

I now pass on to the experiments described in the paper and the inferences 
drawn from them. 

Let us in the first place inquire whether the fact of heating observed by 
Professor Magnus implies such condensation as must of necessity play the 
important part assigned to it by Professor Magnus. 

The question may be answered experimentally. A thin plate of highly 
polished silver was laid agaiust the face of a thermo-pile. Pirected towards 
the plate was a glass tube connected with a holder of hydrogen. The gas was 
forced through two drying tubes filled with fragments of glass moistened by 
sulphuric acid, and caused to impinge in a gentle current against the silver. 
The needle of the galvanometer was at the same time observed, and at the 
beginning of the experiment pointed to a high figure, indicating that the face 
of the pile in contact with the plate was colder than the opposite one. 

The moment the dry gas touched the plate of silver the needle moved to a 
still higher figure, declaring, therefore, the further chilling of the pile. But 
an opposite action immediately set in. J he needle stopped, returned, tell 
rapidly to zero, and went up to 90° on the other side of it. It could, in fact, ^ 
be made to swing quite round the dial by the generated heat. 

A highly polished plate of brass was then substituted for the silver. Oa 
causing a jet of dry hydrogen to impinge against it, precisely the same effects 
were observed. The needle, as before, could be swung quite round its dial 
by the heat generated through the contact of gas and metal. 

Dry air, or dry carbonic acid, produced no heating of this kind, but, on the 
contrary, the. chill already observed by Professor Magnus. 

When the jet of hydrogen continued to play upon the plate of brass for a 
considerable time, the needle slowly sank towards zero, and went up to its 
original position on the side of cold. The attraction which produced the first 
heating being satisfied, either the attracted film remained there, the heat pro¬ 
duced "by it being dispersed, or it was incessantly displaced, the chill of its 
• evaporation balancing the heat of condensation. Probably, however, the film, 
once seized upon by the surface, remained attached, the succeeding gushes of 
the gas gliding over it without coming into any real contact with the metal.. 

WhatTthenls the effect of this powerful condensation upon the transmission 
of radiant heat through a polished brass experimental tube? None whatever. 
Oxyo-en is inactive, hydrogen is inactive, the mixture of oxygen and hydrogen 
is inactive. It will be remembered that Professor Magnus infers both the 
existence of the liquid film, and the absorption by the film, from the fact of 
heatin°\ The foregoing experiments prove that the inference is not necessarily 
correct" that we may have heating, probably more powerful than that observed 
bv Professor Magnus, without anv sensible interference with the transmission. 

Let us now pa°ss on to vapours. Not only does aqueous vapour produce the 
observed heating effect, but alcohol, according to Professor Magnus, produces 
it in a much greater degree. This he first infers from the energy of its 
abaorntion and he afterwards confirms the inference by direct experiment. 
In a former paper he had indeed shown that ‘results perfectly similar to 
those obtained with vapour of water were also obtained with the vapour of 

alcohol, of ether, and other vapours.’ * . 

Now, in the first memoir of this series, which was written twelve years ago, 


* Phil. Mag. vol. xxvii. p. 249, 


394 REMARKS ON PROFESSOR MAGNUS’S LAST PAPER. 


it is stated that the fear lest a change of the reflecting surface of my experi¬ 
mental tube might have some share in the production of the observed effects, 
caused me to compare the deportment of all the vapours in a polished tube with 
their action in a blackened one. The substantial identity observed in the two 
cases renders the conclusion impossible that condensation on the surface of the 
polished tube could have had any material influence on the results. 

But let us give the argument its entire logical value. It may be urged that 
though the influence of the interior surface of the tube is proved practically 
nil by these experiments, the surfaces of the rock-salt remained, and the liquid 
films may have collected on them. A conclusive reply to this criticism is given 
by the numerous experiments recorded in Memoir V. For were the measured 
absorptions due to films on the plates of salt, and not to the vapour between 
them, the mere augmentation of the distance between the plates would produce 
no augmentation of the absorption. But that the reverse is the truth is amply 
established in Memoir V. The case of sulphuric ether will suffice. It was 
measured in layers varying from 005 of an inch to 2 inches in thickness, 
the absorption rising with the distance traversed by the rays from 2 per 
cent, to 35 per cent, of the total radiation. 

It is thus, I think, established that the particular kind of surface conden¬ 
sation invoked by Professor Magnus, and which consists of a film, not only 
invisible to ordinary sight by ordinary light, but which I can affirm to be 
invisible even when extraordinary means are taken to reveal it, has no 
disturbing influence on the reflexion from the interior polished surface of 
a brass experimental tube. 


Latest Experiments. 


The tube with which the polished one was compared, was, as stated above, coated 
with lamplack, but a still more conclusive series of experiments remains to be 
mentioned. An experimental tube 38 inches long and 6 inches in diameter 
had two ends attached to it, each perforated by an aperture 2 6 inches in 
diameter. These apertures were closed with plates of rock-salt. The source 
of heat was a platinum spiral, well defended from air-currents, and heated 
to redness by an electric current. In front of the spiral wa9 a rock-salt lens, 
which sent a slightly convergent beam through the tube. Behind the most 
distant plate was formed a sharply defined image of the spiral, its size being 
such that it teas wholly embraced by the plate of salt. Here then was a beam 
of heat passing through an experimental tube without cominy into contact 
either with the surface of the tube itself, or until any coating or lining of that 
surface. With this apparatus all my old experiments on vapours have been 
frequently repeated. There is no substantial difference between the results thus 
obtained, and those obtained with an experimental tube, where nineteen- 
twentieths of the heat which reached the pile, was reflected heat. If, therefore, 
a film be deposited on the interior surface, and if its action be at all sensible, 
that action must be precisely proportional to the action of the pure vapour, 
and can therefore introduce no disturbance into comparative measurements. 

I may add that experiments made with the same tube, source of heat, and 



LATEST EXPERIMENTS. 


395 


lens entirely confirm the results announced in Memoir VI., where it is shown 
that the order of absorption of liquids and their vapours is the same. 

Experiments have been also made with this wide experimental tube on dry 
and moist air. I had hazarded the estimate that 10 per cent, of the earth s 
radiation is absorbed within ten feet of the surface, and I wished to test 
this surmise with a source of heat whose rays should closely resemble in quality 
the earth's emission. This, at all events in England, is mainly derived from 
moist surfaces: the radiation from water thus forming a principal part of the 
earth’s emitted heat. From experiments recorded in Memoir VI. it maybe 
safely inferred that the radiation of a hydrogen flame is similar in quality to 
that of water. This, therefore, was chosen as a source of heat in applying the 
test referred to. 

Dry and moist air in succession were permitted to enter the experimental 
tube, the sides of which as before, were entirely withdrawn irom the radiation. 
A long series of concurrent experiments yielded the mean result that the 
aqueous vapour of a column of humid air, a yard in length, intercepted 8 pet 
cent, of the total radiation from a hydrogen flame. The tube was, as usual, 
stopped with plates of rock-salt, which on being detached showed not the 
slightest trace of moisture. 

The air here entered the experimental tube through a stopcock placed at 
its centre. .Removing the plates of salt, and gently forcing dry air and moist 
successively into the tube, the difference between them amounted to 5 percent, 
of the total radiation. 

But a noteworthy circumstance is now to be mentioned. To an obsei\er 
looking at the needie as the moist air entered the closed experimental tube, the 
action°of the aqueous vapour would in some cases appear to be absolutely nil. 
At starting and at concluding the needle would point to zero, or nearly so. 
But when this was the case it was always found that the entrance of dt y air 
caused a deflection of seven or eight degrees on the side of heat. This was 
entirely due to the dvnamic heating of the interior surface of the tube by the 
collision of the air j and this effect, plus the slightly additional effect of surface 
condensation, had to be overcome by the moist air. In short, when the needle 
remained at zero with the moist air, the deflection produced by the dynamic 
heating of the dry air became the measure of the absorption. 

This fact is capable of application. With a lining of smooth pasteboard and 
moist air Professor Magnus found his needle to remain motionless. Does this 
prove the absence of absorption P No more than the case just cited. M;iy it 
not be that the warming of the pasteboard tube exactly made good the absorp¬ 
tion P Professor Magnus begins with a lightly coated tube and finds the 
absorption fall from 3-75 to 1*4. With smooth pasteboard tube he finds the 
absorption nil, while with a tube more thickly coated than the first 
he crosses the zero of neutrality, and finds the pile heated instead of chilled. 
Is it to be assumed that the heating thus manifested in the third tube was 
entirely absent from the first? If not the figure P4 does not express the 
absorption, which ought to be 1-4 plus the radiation from the interim surface. 
With the thickly coated tube the increase through surface radiation amounted 
to 1 per cent; and when no external source of heat was employed the 
increase was still greater. How much greater is not stated ; and we are left 
without the means of analysing the composite effect produced by the true 
source of heat and the warmed inteiioi suiface. 


CONCLUDING REMARKS AND SUMMARY. 


I nAYE thus endeavoured to unravel a very tangled skein, and I think these 
Memoirs testify that I have not shrunk from the necessary labour. It may, 
moreover,*be taken for granted that the work here recorded constitutes but a 
small fraction of that really done. Indeed scientific literature is so voluminous, 
and human life so short, that I thought it right to confine myself to the bare 
essentials of experimental evidence. 

Professor Magnus and myself approached this question from different points 
of view. He had not experimented with vapours: I had. lie had not varied 
the density of the more powerful gases : I had. The consequence was, that I 
approached aqueous vapour thinking that it would be found to exert a sensible 
action, while he approached it with the certainty that it would exert no action.* 
Both expectations were fulfilled; he found the action of aqueous vapour ml, 
while I found it to be 15 times that of the air in which it was diffused. By 
purifying the air and thus lowering its action, the absorption of aqueous vapour 
became subsequently 30, 40, 50, even 90 times that of the pure air with which 
it was mixed. 

Professor Magnus first explained the discrepancy between us by ascribing my 
results to the condensation of moisture in the liquid form on my plates of rock- 
salt. By special experiments he proved it possible to render rock-salt dripping 
wet in a sufficiently humid atmosphere. It was a matter of everyday experi¬ 
ence with me that such a result was possible ; but I replied by showing that 
my plates of salt were as dry during my experiments as plates of glass or rock- 
crystal. After experimenting with moist air I submitted the plates of salt to 
Professor Magnus himself, and he could detect no trace of moisture upon them. 

The objection to the rock-salt plates was still further met by abolishing 
them altogether, and showing that precisely the same results could be ob¬ 
tained with a tube open at both ends. These results were shown to Professor 
Magnus, but he did not note whether the deflections which I brought under his 
notice were due to heat or cold ;t mid on repeating the experiments he obtained 
deflections similar to mine in magnitude, but opposite in direction. His results 
were proved to be due to the condensation of aqueous vapour on the face of 
his pile when humid air was urged against it, and to the evaporation of the 
condensed moisture when dry air was urged against the pile. 

I was perfectly well acquainted with the source of error pointed out by 
Professor Magnus, and, warned by this knowledge, I avoided the error. But 
I did not content myself with reaffirming the correctness of my results. The 
apparatus was handed over to Dr. Frankland, who, having entire control of 
it, repeated my experiments with critical care. He verified them all. They 
were subsequently proved true by Professor Wild, and still later by Professor 
Magnus himself. 

* ‘Obgleich mit Bestimmtbeit vorauszusehen war, class die geringe Menge von Wasser- 
dampf, welche die Luft bei gewohnlicher Temperatur autzunehmen vermag, vou keinem 
Einfluss auf die Durchstrahlung sein konne.’— Pogg. Ann, vol. cxii. p. 539. 

| Phil. Mug. vol. xxvi. p. 23. 


CONCLUDING REMARKS AND SUMMARY. 


397 


In conversation he had mentioned to me that the mechanical impurities of 
London air might be the cause of the action which I had ascribed to aqueous 
vapour. • I replied by operating on air from Epsom Downs, the Isle of Wight, and 
other places. London air, moreover, was purified until it became neutral, and 
afterwards, without coming into contact with any source of impurity whatever, 
was led over fragments of clean glass moistened with distilled water. Thus 
charged with vapour, its action was proved even greater than that of pure 
country air. 

It was intimated to me as early as 1862 that condensation on the interior 
surface of the experimental tube might, by diminishing the reflexion, have 
produced my results. This source of error was examined by Professor Magnus 
in 1863, and abandoned.* I replied to the objection, when first mentioned, 
by showing that the absorption was accurately proportional to the quantity of 
humid air present in the experimental tube, a result almost physically im¬ 
possible to be produced by condensation. 

The quality of the evidence was then varied. Comparing the absorption of 
liquids with that of their vapours, both were found with follow the same order. 
In no case whatever, where the volatility of the liquid is sufficient to render 
its vapour manageable, has an exception to this law been discovered. The posi¬ 
tion of a liquid, therefore, as an absorber of radiant heat, fixes practically that of 
its vapour; and as water has been proved by Melloni to be of all liquids the 
most energetic absorber of radiant heat, it may be safely concluded that the 
vapour of water has a corresponding power. 

So constant is the relation between vapourous and liquid absorption that in the 
various and singular shiftings of diathermic position revealed in Memoir VI. the 
vapour followed with undeviating precision the fluctuations of the liquid from 
which it was derived. 

Nor is this constancy of relation confined to the thermal rays of low refran- 
gibility. In the experiments on rays of high refrangibility recorded in Memoir 
VII. it is shown that the ‘ penetrability ’ of liquids to the chemical rays has 
its exact parallel in the penetrability of their vapours to the same rays. 

Such facts prove, I think to demonstration, both radiation and absorption to 
be, in the main, the acts of the constituent atoms of the molecules of compound 
bodies. For were they to any great extent the acts of the molecule taken as 
a whole, the change from the gaseous to the liquid state, altering so pro¬ 
foundly the relations between molecule and molecule must introduce changes 
in the order of absorption. If this be true, then the liberation of the molecules 
of water from the liquid condition, cannot destroy the radiant and absorbent 
power of the constituent atoms; though the unparalleled attenuation of aqueous 
vapour must enormously diminish the action. (At page 428 this subject is 
further considered). 

To this attenuation, in the first place, to the thickness of the strata employed, 
and to the fact that the experiments were conducted in air already charged 
to a great extent with aqueous vapour, the negative results obtained by Pro¬ 
fessor Magnus in his radiation experiments are, in my opinion, to be ascribed, t 


* Phil. Mag. vol. xxvi. p. 23. 

f In these experiments Professor Magnus carried his air through water. Even then the 
quantity of vapour taken up is very small. The volatility of a liquid, at all events the ease 
with which it yields its vapour to dry air, is by no means indicated by the boiling point of the 
liquid. The quantity of aqueous vapour taken up by dry air is a minimum. Other liquids 


398 


CONCLUDING REMARKS AND SUMMARY. 


Further evidence regarding the absorption of heat by aqueous vapour, based 
upon the principle that the emission from any vapour is absorbed with 
peculiar avidity by that vapour, is adduced. It is proved in Memoir VII. that, 
despite enormous differences of temperature between the radiant and the absor¬ 
bent, a gas or vapour is intensely opaque to the radiation from itself. Thus car¬ 
bonic acid, one of the feeblest absorbers of heat from ordinary sources, transcends 
all other gases in its action on the heat emitted by a carbonic-oxide flame. 
Hence the fair conclusion that aqueous vapour will be found specially opaque 
to the emission from a hydrogen flame. This inference of reason is completely 
confirmed by experiment. When a hydrogen flame is the source of heat, the 
absorption is treble that exerted on the emission from a platinum wire, with a 
temperature not higher than it assumes when plunged into a flame of 
hydrogen ; while the actual immersion of such a wire so changes the quality 
of the heat as to reduce the absorption to one-half that exercised upon the 
emission from the flame above. 

To Professor Magnus’s latest experiments, in which he seeks to show that 
my results were due to the formation of a broken layer of water on the 
interior surface of my experimental tube, I reply by employing an expe¬ 
rimental tube, with a diameter two and a half times that of the plates 
of salt used to close it, sending through these plates a beam which touches 
neither the surface of the ttihe, nor any lining used to clothe that surface, and 
proving by this crucial test that, not only aqueous vapour, but all the other 
vapours, behave exactly as they had been found to behave in a tube which 
made reflected heat nineteen-twentieths of the total radiation. 

Finally, the phenomena of meteorology all speak in favour of the action 
ascribed to aqueous vapour. The great thermometric range in the interior of 
Australia; the night chill of Sahara ; the temperature of the table-land of Asia; 
the great fluctuations observed by Hooker in the Himalayas, and a multitude of 
similar effects observed at other places, are perfectly accounted for by the 
alleged action. To this evidence Professor Magnus offers the single reply, that 
the checking of the terrestrial drain of heat is due to 1 nebulous’ instead of 
vaporous matter. When Leslie affirms that his cethrioscope on days equally clear 
indicates widely different degrees of terrestrial radiation, Professor Magnus 
replies that the days are not equally clear. The same reply is offered to 
Stracliey’s observations. The meteorologist vainly appeals to the visible purity 
of the sky; when the firmament is of the deepest blue there is still unseen 
cloudy matter in the air, and to it is due the stoppage of terrestrial radiation. 

It is needless to say that, if we except the experiments of Professor Magnus, 
which have just been subjected to analysis, there is not the slightest warrant 

with boiling points far higher than that of water yield far greater quantities of vapour to 
dry air passing through them. They are more speedily exhausted, and they evaporate more 
quickly when exposed in the common air. A great number of experiments illustrating 
this point have been executed. Glass fragments, placed in two U-tubes, and moistened 
with equal weights of iodide of allvl (boiling point 101° C.) and water, had the same quan¬ 
tity of dry air carried over them. The loss by evaporation of the iodide was 97 per cent., 
while the loss of water was only 19 per cent, of the total weight. Toluol (boiling point 
110-3° C.) and water treated in the same manner showed a loss of 88 per cent, of the 
former to a loss of only 14 per cent, of the latter. The nitrite of amyl with a boiling 
point of 9<° C. showed a loss of 90 per cent, as contrasted with a loss of 12 per cent, by 
water. Even the nitrate of amyl with' a boiling point of 149° C. is taken up in far 
greater quantities by dry air than water is. 


ICE-MAKING IN THE TROPICS. 


399 


for this assumption. My experiments lead me to conclude that the cloudy 
matter capable of producing the observed effects would infallibly be de¬ 
tected. As stated in Memoir VI., the dry smoke of London is found to 
exert but a small comparative action on radiant heat. Such smoke if raised 
aloft in the atmosphere would assuredly sully the purity of the sky. And if, 
for every particle of carbon, a particle of water were substituted, the effect upon 
the firmamental blue could not, in my opinion, escape observation. This, 
however, is a matter which can be completely set at rest by meteorologists; 
for it, during days of palpable firmamental impurity and great dryness, the 
radiation should be found greater than on humid days with an unsullied azure 
overhead, the assumption will be left without any basis. 

There is, I believe, proof that in India and Australia the radiation is often 
greater when the sky is dimmed with dust, than when it is visibly free from 
floating matter. It is not the dry winds which bring this dust that check the 
radiation, but the humid winds which carry no dust but render the sky clear. 


Bearing upon this subject is an article on Ice-making in the Tropics, recently 
published in ‘ Nature ’ by Mr. Wise. I hope the author will allow me to show 
my interest in his communication by reproducing the greater part of it here. The 
care taken to secure local dryness is very manifest. The ground is first dried 
by the sun, and the straw which covers the ground and on which the dishes 
rest, is also carefully kept dry. A glance at the map of India will explain 
the efficacy of the N.N.VV. wind, and the hindrance offered by the southerly 
and easterly winds to the formation of ice. In the former case the air not 
only crosses the hills mentioned by Mr. Wise, but in all probability has rolled 
over the Himalayas from Thibet; in the latter case it comes from the adjacent 
ocean. The thermometric difference between 48°, the temperature of the air 
a yard above the straw-beds, and 27°, the temperature of the beds themselves, 
is greater than I had supposed it to be. It is also interesting to know that the 
wind which resists the formation of ice is often colder than the one which 
favours it. 


Ice-making in the Tkopics. 

By J. A. Wise. 

The following method is employed by the natives of Bengal for making ice 
at the town of Hooghly near Calcutta, in fields freely exposed to the sky, and 
formed of a black loam soil upon a substratum of sand. 

The natives commence their preparations by marking out a rectangular piece 
of ground 120 feet long by 20 broad, in an easterly and westerly direction, 
from which the soil is removed to the depth of two feet. This excavation is 
smoothed, and is alloiued to remain exposed to the sun to dry , when rice straw 
in small sheaves is laid in an oblique direction in the hollow, with loose straw 
upon the top, to the depth of a foot and a half, leaving its surface half a foot 
below that of the ground. Numerous beds of this kind are formed, with 
narrow pathways between them, in which large earthen water-jars are sunk in 
the ground for the convenience of having water near, to fill the shallow un¬ 
glazed earthen vessels in which it is to be frozen. These dishes are 9 inches 



400 


ICE-MAKING IN THE TROPICS. 


in diameter at the top, diminishing to 4— inches at the bottom, 1~ deep, and 
t 3 5 of an inch in thickness; and are so porous as to become moist throughout 
when water is put into them. 

During the day the loose straw in the beds above the sheaves is occasionally 
turned up, so that the whole may be kept dry, and the water-jars between the 
beds are tilled with soft pure water from the neighbouring pools. Towards 
evening the shallow earthen dishes are arranged in rows upon the straw, and 
by means of small earthen pots, tied to the extremities of long bamboo rods, 
each is filled about a third with water. The quantity, however, varies accord¬ 
ing to the expectation of ice—which is known by the clearness of the sky, and 
the steadiness with which the wind blows from the N.N.W. When favourable, 
about eight, ounces of water is put into each dish, and when less is expected, 
from two to four ounces is the usual quantity ; but, in all cases, more water is 
put into the dishes nearest the western end of the beds, as the sun first falls on 
that part, and the ice is thus more easily removed, from its solution being 
quicker. 

There are about 4,590 plates in each of the beds last made, and if we allow 
five ounces for each dish, which presents a surface of about 4 inches square, 
there will be an aggregate of 239 gallons, and a surface of 1,530 square feet of 
water in each bed. 

In the cold season, when the temperature of the air at the ice-fields is under 
50° F., and there are gentle airs from the northern and western direction, ice 
forms in the course of the night in each of the shallow dishes. Persons are 
stationed to observe when a small film appears upon the water in the dishes, 
when the contents of several are mixed together, and thrown over the other 
dishes. This operation increases the congealing process; as a state of calmness 
has been discovered by the natives to diminish the quantity of ice produced. 
When the sky is quite clear, with gentle steady airs from the N.N.W., which 
proceed from the hills of considerable elevation near Bheerboom, about 100 
miles from Ilooghly, the freezing commences before or about midnight, and con¬ 
tinues to advance until morning, when the thickest ice is formed. I have seen 
it seven-tenths of an inch in thickness, and in a few very favourable nights the 
whole of the water is frozen, when it is called by the natives solid ice. When 
it commences to congeal between two and three o’clock in the morning, thinner 
ice is expected, called paper-ice; and when about four or five o’clock in the 
morning, the thinnest is obtained, called flower-ice. 

Upwards of two hundred and fifty persons, of all ages, are actively employed 
in securing the ice for some hours every morning that ice is procured, and this 
forms one of the most animated scenes to be witnessed in Bengal. In a 
favourable night upwards of 10 cwt. of ice will be obtained from one bed, and 
from twenty beds upwards of 10 tons. 

When the wind attains a southerly or easterly direction, no ice is formed, 
from its not being sufficiently dry ; not even though the temperature of the 
air be lower than when it is made w'ith the wind more from a northern or 
western point. The N.N.W. is the most favourable direction of wind for 
making ice, and this diminishes in power as it approaches the due north, or 
west. In the latter case more latitude is allowed than from the N.N.W. to 
the north. So great is the influence of the direction of the wind on the ice, 
that when it changes in the course of a night from the N.N.W. to a less 
favourable direction, the change not only prevents the formation of more ice, 


ICE-MAKING IN THE TROPICS. 


401 


but dissolves wliat may have been formed. On such occasions a mist is seen 
hovering over the ice-beds, from the moisture over them, and the quantity 
condensed by the cold wind. A mist in like manner forms over deep tanks 
during favourable nights for making ice. 

Another important circumstance in the production of ice is the amount of 
wind. When it approaches a breeze no ice is formed. This is explained by 
such rapid currents of air removing the cold air, before any accumulation of 
ice has taken place in the ice-beds. It is for these reasons that the thickest ice 
is expected when during the day a breeze has blown from the N.W., which 
thoroughly dries the ground. 

The ice-dishes present a large moist external surface to the dry northerly 
evening air, which cools the water in them, so that, when at 61°, it will in a 
few minutes fall to 56°, or even lower. But the moisture which exudes 
through the dish is quickly frozen, when the evaporation fr6m the external 
surface no longer continues radiative; a more powerful agent then produces 
the ice in the dishes. 

The quantity of dry straw in the ice-beds forms a large mass of a bad con¬ 
ductor of heat, which penetrates but a short way into it during the day; and 
as soon as the sun descends below the horizon, this large and powerfully-radi¬ 
ating surface is brought into action, and affects the water in the thin porous 
vessels, themselves powerful radiators* The cold thus produced is further 
increased by the damp night air descending to the earth’s surface, and by the 
removal of the heating cause, which deposits a portion of its moisture upon 
the now powerfully radiating and therefore cold surface of the straw, the 
water, and the large moist surface of the dishes. When better radiators [P] of 
heat were substituted, as glazed white, or metallic dishes, the cold was greater, 
and the ice was thicker, and the dishes were heavier in the morning than the 
common dishes. Any accumulation of heat on their surface from the deposit of 
moisture is prevented by the cold dry north-west airs which slowly pass over 
the dishes. The winds quickly dries the ground, and declines towards night 
to moderate airs. The influence of these causes is so powerful'that I have seen 
the mercury in the thermometer placed upon the straw between the dishes 
descend to 27°, when three feet above the ice-pits it was 48°. 

So powerful is the cooling effect of radiation on clsar nights in tropical 
climates, that in very favourable mornings, during the cold season, drops of 
dew may sometimes be found congealed in Bengal upon the thatched roofs of 
houses, and upon the exposed leaves of plants. In the evening the cooling, 
process advances more rapidly than could be supposed by one who has not 
experienced it himself, and proves the justness of his feelings, by the aid of 
the thermometer. In the open plain on which the ice is made, I have seen the 
temperature of the air, four feet above the ground, fall from 70-5° to 57°, in 
the time the sun took to descend the last two degrees before his setting. 


[Here follow a feiv shorter papers which are either referred to in the 
foregoing Memoirs , or contain something which the Memoirs do not 

contain.'] 


26 



































* 
















































































































































XII. 


RECENT RESEARCHES ON -RADIANT HEAT. 









































XII. 


RECENT RESEARCHES ON RADIANT HEAT* 

The last number of Poggendorff’s c Annalen’ contains a short 
paper by Professor Magnus, ‘ On the Passage of Radiant Heat 
through Moist Air,’ a translation of -which appears in the 
present number of the ‘ Philosophical Magazine.’ This paper 
has excited considerable interest and some discussion among: 
the scientific men of London, and it is on many accounts 
desirable that I should not delay attempting to offer an expla¬ 
nation of the differences which exist between my eminent friend 
and myself. A brief sketch of the history of the subject is also 
considered desirable; and this, as far as the extremely limited 
time at my disposal will admit of, I shall also endeavour to 
supply. 

On the first perusal of Melloni’s admirable work e La Ther- 
mochrose,’ which came into my hands soon after its publication, 
the thought of investigating the action of gases on radiant heat 
occurred to me. Melloni, it will be remembered, failed to 
obtain any evidence of the absorption of radiant heat by a 
column of atmospheric air 18 or 20 feet long. My attention 
was further fixed upon this subject by the discussion carried on 
in 1851 between Professors Stokes and Challis, regarding 
Laplace’s correction for the theoretic velocity of sound. Pro¬ 
fessor Challis, it will be remembered, contended that Laplace 
had no right to his correction, because the heat evolved in con¬ 
densation would be instantly wasted by radiation in a mass of 
air of indefinite extension. In the first lecture of my first 
course at the Royal Institution in 1853, the compression of air 
in a rock-salt syringe was proposed to decide the question ; and 
in a paper presented quite recently to the Royal Society, this 

* Philosophical Magazine , for April, 1862. This paper is referred to at the end 
of the ‘ Historic Remarks’ on Memoir I. 


406 


RECENT RESEARCHES ON RADIANT HEAT. 


point lias been solved in a manner which I hope Professor Challis 
himself will deem conclusive, the mode of solution resembling' 
in some respects the device of 1853. In 1854 the action of 
gases and vapours on radiant heat w T as a frequent subject of 
conversation between my scientific friends and myself; and 
some of these still remember my remarks at the time, the hopes 
entertained regarding the subject, and the devices by which it 
was proposed to meet its difficulties. I was, however, prevented 
by other engagements from attacking the subject at this time; 
and not till the early spring of 1859 were my ideas brought to 
practical definition. Then, however, I devised and applied the 
apparatus which, with some modifications and improvements, 
has been used ever since. 

This apparatus immediately opened to rue a large and rich 
field of experimental inquiry; and the greatest pleasure this 
discovery gave me, and which was often expressed to Mr. Fara¬ 
day at the time, was, that it placed me in possession of a subject 
in the prosecution of which I could not possibly interfere with 
the claims of any previous investigator. The first notice of 
my researches is published in the ‘ Proceedings of the Royal 
Society 5 for May 26, 1859. On June 10 following, they were 
made the subject of a Friday evening discourse at the Royal 
Institution. The late lamented Prince Consort was present on 
this occasion, and with characteristic kindness interested him¬ 
self afterwards to obtain plates of rock-salt for me. I then 
executed many of my experiments in presence of a large 
audience ; and an account of the discourse is published in the 
‘ Proceedings of the Royal Institution ’ of the date referred to. 

I also communicated an account of the investigation to my 
friend Professor de la Rive, and he published a translation of 
my letter in the Bibliotheque Universelle. The investigation was 
also described in Cosmos , in the Nuovo Cimento , and in other 
journals. When I reached Paris in 1859, the subject had 
attracted a greater degree of attention than I could have hoped 
to see bestowed upon it. In short, the publicity of my mode of 
experiment and results was quite general. 

I will here ask permission to cite a number of these results 
obtained during the month of July 1859, after the main 
difficulties of my apparatus had been surmounted. [They are 
certainly closer to the truth than those published in 1861 by 


RECENT RESEARCHES ON RADIANT HEAT. * 


407 


Professor Magnus.] The method employed was substantially 
the same as that described in my last memoir.* The heat 
passed from the radiating surface through a vacuum into the 
experimental tube; the principle of compensation was also 
employed ; the length of the tube used to receive the gas was 12 
inches; and from the galvanometric deflection consequent on the 
admittance of the gas or vapour its absorption was deduced. 


Name of gas 

I. 

Gases. 

Deflection 

Atmospheric air . 



. 6°. 

Oxygen .... 



. 8°; 8°; 7°; 7°. 

Nitrogen. 20th July . 



. 6°; 5°. 

Again, 25th July 



• 

O 

o 

• 

Hydrogen .... 



. 10°; 10°. 

Carbonic oxide . . 



. 34°; 34°; 34°. 

Carbonic acid 



. 37-5°; 35°; 37’5°; 37°. 

Nitrous oxide 



. 67-5°; 57-5°; 57'5°. 

Olefiant gas, 1 inch pressure 



. 43°; 43°. 

„ ,, 5 inches „ 



. 62*5°; 62-5°. 

„ „ 30 inches „ 



. 74°. 

Coal-gas, 1 inch pressure 



. 28°. 

,, 5 inches ,, 



O 

CO 

0 

„ 30 inches „ 



o 

o 

Total heat .... 



. 79-8°. 


The figures separated from each other by semicolons indicate 
the results of different experiments ; and their close agreement 
shows the accuracy which, even in this early stage of the in¬ 
quiry, the experiments had attained. The above deflections 
represent the following absorptions, at a common pressure of 
30 inches of mercury :— 


II. Gases. 


Name of gas 

Absorption per 100 

Name of gas 

Kelative Absorption 

Atmospheric air 

. 6 

Carbonic acid. 

. 37 

Nitrogen 

6 

Nitrous oxide. 

. 110 

Oxygen . 

. 7 

Olefiant gas . 

. 345 

Hydrogen 

. 10 

Coal-gas 

. . 345 

Carbonic oxide 

. 34 



The vapours of 

the following substafices were 

also examined 


in the same month, at a common pressure, and the annexed 
results were obtained. 

* Philosophical Transactions , February 1861 ; Philosophical Magazine , Sept. 1861. 












408 . RECENT RESEARCHES ON RADIANT IIEAT. 


III. Vapours. 


Name of vapour 



Deflection 

Bisulphide of carbon 


• 

16°; 17°. 

Bichloride of carbon 


• 

33°; 33°. 

Iodide of methyl 


• 

37-5°; 37-5°. 

Chloroform .... 


• 

*>- 

o 

o 

te. 

i—. 

o 

o 

o 

• 

Benzol. 


• 

43°; 43°; 44°. 

Amylene .... 


• 

55°; 55°. 

Wood-spirit .... 


• 

55°; 55°. 

Methylic alcohol (from Dr. W.) 


• 

55-5°; 55°. 

„ „ (from Dr. H.) 


• 

63-5°; 64° (impure). 

Ethylic ether .... 


• 

63-5°; 63°. 

Absolute alcohol 


• 

64-5°; 64*5°. 

Ethyl-amylic ether . 


• 

65°; 65°. 

Sulphuric ether 


• 

67°; 67°. 

Propionate of ethyl 


• 

68°: 68°. 

Acetate of ethyl . 


• 

O 

o 

-4 

O 

o 

Double brass screen 


• 

79-8°. 


I \ 

These deflections correspond to the following absorptions, 
omitting decimals:— 


IY. Vapours. 

Name of vapour Absorption per 100 Name of vapour Relative Absorption 


Bisulphide of carbon 

. 17 

Ethylic ether . 

. 200 

Bichloride of carbon 

. 33 

Absolute alcohol 

. 210 

Iodide of methyl 

. 38 

Ethyl-amylic ether . 

. 216 

Chloroform 

. 44 

Sulphuric ether 

. 237 

Benzol 

. 50 

Propionate of ethyl. 

. 252 

Amylene . 

. 84 

Acetate of ethyl 

. 282 

Pure methylic alcohol 

. '84 



These results, winch followed many thousand undescribed ex¬ 
periments, were all obtained before the end of July 1859 ; and I 
should certainly have published them and many others in extenso 
at the time, had I not felt that the wide circulation the general 
description of the inquiry had obtained relieved me from this 
necessity. I wished to impart the last finish to my apparatus, 
and to pursue the subject with that deliberation and thorough- 
ness which its difficulty and importance demanded. Not until 
the close of 1860 was the full account of the investigation drawn 
up; and the memoir in which it was embodied bears the receipt 
of the Royal Society for January 10, 1861. It afterwards 
formed the Bakerian Lecture for the year. 

For months I was harassed by the discordant results obtained 
with gases generated in different ways. The nitrogen obtained 










RECENT RESEARCHES ON RADIANT HEAT. 


409 


from the passage of air over heated copper turnings gave me at 
first many times the effect of the air itself; that obtained from 
the combustion of phosphorus in air differed from both; while 
the nitrogen obtained from the nitrate of potassa could not 
be made to agree with its fellows. In like manner, the oxygen 
obtained from the chlorate of potash and peroxide of manganese 
differed from electrolytic oxygen; the hydrogen obtained from 
sulphuric acid and ztfnc differed from electrolytic hydrogen; the 
carbonic oxide obtained from chalk and carbon differed from 
that generated from the ferrocyanide of potassium, while 
carbonic acid from different sources showed similar anomalies. 
It will be borne in mind that at this time nothing whatever teas 
Imoivn of the vast action which a small amount of certain impuri¬ 
ties can exert , and that my own experiments were the first to exhibit 
this action. 

Further, my drying apparatus first consisted of sixteen feet of 
glass tubing filled with chloride of calcium, and a large U-tube 
filled with fragments of pumice-stone moistened with sulphuric 
acid. Sometimes the chloride of calcium was used alone, some¬ 
times the sulphuric acid, and sometimes both were used toge¬ 
ther. Every morning it was necessary to allow the air to pass 
through the drying apparatus, and fill the experimental tube 
seyeral times before the results became constant; and even after 
they had become tolerably constant with the chloride of calcium, 
the introduction of the sulphuric acid caused a considerable 
variation of the absorption. This might naturally be ascribed 
to the more perfect desiccation of the air by the acid, but this 
does not account for the effects obtained. For when both were 
used, the magnitude of the absorption was found to depend on 
the circumstance whether the air entered the sulphuric-acid tube 
or the chloride-of-calcium tube first. I will here give an example 
of this irregularity :— 

Absorption 


Air passed through Ca Cl alone.7 

When SO 3 was added.4 

Through new Ca Cl tube ......... 7 

New SO 3 tube added.4 

Through another Ca Cl tube alone.7 

A fresh tube of SO 3 added ........ 5 

Reversed current of air, and sent it through SO 3 first . . .10 

t 


The fluctuations above referred to are here distinctly ex- 








410 


RECENT RESEARCHES ON RADIANT HEAT. 


hibited; and the last experiment shows that, without changing 
the tubes in any way, but merely by reversing the direction in 
which the current of air passed through them, the absorption 
was doubled. Difficulties almost innumerable of this kind had 
to be overcome. I finally abandoned the chloride of calcium and 
the pumice-stone altogether, and made use of fragments of pure 
marble for my caustic potash, and of pure glass for my sulphuric 
acid. But with these also a long time elapsed before I was 
master of the anomalies which from time to time made their 
appearance. The dust of a cork ; a fragment of sealing-wax, 
so minute as almost to escape the eyesight; the moisture of the 
fingers touching the neck of the U-tube, in which the sulphuric 
acid was contained—these, and many other apparently trivial 
causes, were sufficient entirely to vitiate the results in delicate 
cases, giving me on many occasions effects which I knew to be 
large multiples of the truth. Thus, while perfectly safe as re¬ 
gards the stronger gases whose energy of action masked small 
errors, prolonged experiment was needed to connect these with 
the feebler ones, and to refer them to air as a standard. In 
short, I thought it due both to the public and myself to abstain 
from giving more than a clear general account of the inquiry 
until every anomaly that had arisen had been mastered. I cannot 
regret having exercised this patience, more especially when 
one of the ablest and most conscientious experimenters of 
modern times is found falling into error on some of the points 
which most perplexed me. 

A few weeks subsequent to the receipt of my paper by the 
Royal Society, that is to say, on February 7, 1861, an account 
of experiments on the transmission of radiant heat through 
gases was communicated by Professor Magnus to the Academy 
of Sciences in Berlin. In this inquiry the absorption of heat 
by vapours was left untouched, nor did it embrace the reci¬ 
procity of radiation and absorption which my investigation 
revealed. But as regards absorption by gases, Professor 
Magnus and myself had operated on the same substances; 
and, considering the totally different methods employed, the 
correspondence between our results must be regarded as re¬ 
markable. 

. Previous to occupying himself #vith the transmission of heat 
through gases, Professor Magnus had made an investigation on 


RECENT RESEARCHES ON RADIANT HEAT. 


- 411 


the conduction of heat by gases, and he was led naturally by 
this inquiry to take up the question of gaseous diathermancy. 
My knowledge of his great skill and extreme caution as an ex- 
perimenter entirely ratifies a statement which he has repeated 
more than once in his published memoir, namely, that his re¬ 
sults on the diathermancy of gases were already obtained at the 
time he communicated his results on conduction to the Academy 
of which he is a member, that is to say, in the month of July 
1860. In fact, the very experiments intended to determine their 
conduction really revealed the absorption of the gases. I am 
quite persuaded that the results of Professor Magnus are inde¬ 
pendent of mine, and that, had I published nothing on the sub¬ 
ject, his own inquiries would have led him to the discoveries 
which he has announced. That my researches preceded his by 
more than a year, is simply to be ascribed to the fact of my 
attention having been directed to the radiation of heat through 
gases long before even his researches on conduction had com¬ 
menced. It is needless to dwell upon the value of such a general 
corroboration as that which subsists between Professor Magnus 
and myself. However private interests may fare, science is 
assuredly a gainer when independent courses of experiment 
lead, as in the present instance, to the same important 
results. 

But while furnishing, by an independent method, a highly 
valuable general corroboration of my results, there are some 
special points on which Professor Magnus differs from me; 
and one of these (the action of aqueous vapour on radiant heat) 
he has made the subject of special examination. My first 
exj^eriment gave the action of the vapour of the London air on 
a November day to be 15 times that of the air itself. Only a 
few weeks subsequently Professor Magnus announced, and cited 
very clear experiment^ in support of his statement, that the 
amount of aqueous vapour capable of being taken up by air at 
a temperature of 15 C. has no influence whatever upon the 
absorption. This announcement caused me to repeat my ex¬ 
periments with more than usual care ; and I found the absorp¬ 
tion of the vapour not 15 times, but 40 times that of the air. 
This result was mentioned incidentally in my letter to Sir John 
Herschel; and Professor Magnus, induced by this mention to 
take up the question again, corroborates his former results, and 


412 - RECEXT RESEARCHES OX RADIAXT HEAT. 

finds, by repeated experiments, tliat tbe aqueous vapour of the 
atmosphere lias no influence whatever upon radiant heat, and 
that the rays of the sun, so long as the air is clear, reach the 
earth in the same manner whether the atmosphere is saturated 
with vapour or not.’ 

The more I experiment, the farther I seem to retreat from 
the position of my friend; for in a paper quite recently pre¬ 
sented to the Royal Society, the action of the air of the 
laboratory of the Royal Institution is set down not at 15, nor at 
40, but often at 60 times that of perfectly dry air. In fact, the 
greater my experience and the stricter the precautions I take to 
exclude impurities, the more does atmospheric air, in its action 
on radiant heat, approach the character of a vacuum, and conse¬ 
quently the greater, by comparison, becomes the action of the 
aqueous vapour of the air. 

In the paper which has suggested this communication, Pro¬ 
fessor Magnus assigns as a possible source of error on my part, 
that the aqueous vapour may have been precipitated in a liquid 
form upon my plates of rock-salt. He cites experiments of his 
own to show the hygroscopic nature of this substance ; and 
refers to Melloni’s experiments in proof of the highly opaque 
character of a solution of rock-salt for the obscure rays of heat. 
In a series of experiments made with the express intention 
of wetting the plates of salt by precipitation, Professor Magnus 
exalts the absorption to four times that of air; but though 
the plates were visibly wet, no nearer approach than this 
could be made to my result, which makes the absorption of 
aqueous vapour forty, fifty, and even sixty times that of air. It 
was only on the inner surface of the salt, which came into con¬ 
tact with the saturated air, that the moisture was precipitated 
in the experiments of Professor Magnus; the outer surface, 
which was in contact with the common, air of his laboratory, 
remained dry ; and even the wetted surface, when exposed for a 
time to the same air, became dry also. Now it is with this 
common outer air, and not with air artificially saturated with 
moisture, that I find the absorption of aqueous vapour to be fifty 
or sixty times that of the air in which it is diffused. 

I think it would be hardly possible for a person of any ex¬ 
perimental aptitude whatever to work for three years with 
plates of rock-salt, which must be kept polished and bright, 


RECENT RESEARCHES ON RADIANT HEAT. 


413 


without becoming aware of all the circumstances referred to by 
Professor Magnus. But the truth is that I was well acquainted 
with the peculiarities of rock-salt many years before this investi¬ 
gation commenced.* A slight consideration of the conditions 
of the case will, I think, show how improbable it is that a 
precipitation, such as that surmised, could take place in my 
experiments. First, then, the common air of the laboratory, 
according to Professor Magnus, does not produce the effect which 
he considers may be active in my case; this, as already stated, 
is the air employed in all kinds of weather, dry as well as 
moist. Secondly, this air is introduced into a tube through 
which is passing a flux of heat from the radiating source. 
Thirdly, the air on entering the tube is heated by the stoppage 
of its own motion, and thereby rendered more capable of main¬ 
taining its vapour in a transparent state. The exterior surface 
of my terminal plate of salt was, moreover, always open to 
inspection, and it was never found wet; much less could the 
inner surface be wetted when the laboratory air was used, 
because the temperature within the tube was higher than that 
without. 

But I have not relied on the inspection of the outer surface 
alone of the rock-salt plates. The apparatus has been taken 
asunder more than fifty times, on occasions when I had most 
reason to expect precipitation, but no trace of moisture has been 
found on my plates. 

This, however, did not entirely satisfy me, and I therefore 
made an arrangement of the following kind :—An india-rubber 
bag was filled with air and subjected to gentle pressure. By 
a suitable arrangement of cocks and T-pieces, this air could be 
forced either through a succession of tubes containing fragments 
of marble moistened with caustic potash and fragments of glass 
moistened with sulphuric acid; or through a similar series in 
which fragments of glass were moistened with distilled water. 
A current of either dry air or damp air could be thus obtained 
at pleasure; and my object then was to introduce either the dry 
air or the wet air, under precisely the same conditions, into an 

* The action of moisture upon rock-salt was unhappily made strikingly evident to 
me some months ago; for through a chink in the roof of the laboratory some water 
entered which destroyed two of my plates, and left me more or less a cripple ever 


eince. 


414 


RECENT RESEARCHES ON RADIANT nEAT. 


open tube. To effect this, matters were so arranged that either 
current could be discharged into the same narrow glass tube. 
This glass tube was left in undisturbed connexion with one end 
of my experimental tube, while the other end was connected with 
the air-pump. The plates of salt were entirely abandoned , the 
experimental tube was separated from the ‘front chamber’ 
described in my memoir, and a distance of a foot intervened 
between the radiating surface and the adjacent open end of 
the tube. In front of the other open end of the experimental 
tube was my thermo-electric pile, the c compensating cube ’ being 
applied in the usual way. By pressing the bag and gently 
working the pump, di;y air could, to a great extent, be dis¬ 
placed by moist, and moist air by dry. And in this way, without 
any plates of rock-salt whatever , all the results obtained with 
them were verified. Similar experiments have been executed 
in the case of all other vapours examined: with them, as well 
as with aqueous vapour, my plates of rock-salt are perfectly to 
be relied on. 

Whence, then, the difference between Professor Magnus and 
myself ? I am quite persuaded that no greater care could be 
bestowed upon scientific work than Professor Magnus bestows 
upon his; and it is the perfectly accurate nature of his experi¬ 
ments which renders the explanation of the differences between 
us an easy task. 

Let me, however, first ask attention to what may be called a 
caSe of internal evidence. The mere inspection of the drawing 
of my apparatus ( Frontispiece ) will show that there was a good 
deal of thought and labour expended in its construction. To one 
parfc of it especially I would direct attention. In front of the 
experimental tube is a chamber which is always kept exhausted, 
the radiant heat thus passing through a vacuum into the expe¬ 
rimental tube. To obtain that chamber gave me great trouble: 
it was necessary to unite its anterior wall with silver solder to 
its sides; and this, moreover, had to be done for every special 
source of heat employed. The chamber had also to pass through 
a copper vessel, being soldered water-tight at its place of en¬ 
trance and of exit. This vessel had to be connected by a tube 
20 feet long with the water-pipes of the Institution, so as to 
get a supply; and to carry off the water, I had the stone floor 



RECENT RESEARCHES ON RADIANT HEAT* 415 

of the laboratory perforated, and one of our drains connected 
by a second tube with the vessel. 

As already known, the water-vessel was intended to prevent 
the heat of the source from reaching the first plate of rock-salt. 
To introduce this plate air-tight between the front chamber and 
the experimental tube was also a difficult matter, which required 
special means to meet it. Now, let me ask, what could have 
induced me to go to all this trouble P The .obtaining of suitable 
plates of rock-salt has been one of my greatest difficulties; 
why then did I expend time in seeking for a pair of them ? 
Why did I not content myself with a single plate to stop the 
remote end of my tube, and allow the latter to form a continuous 
whole from the radiating surface to the remote end? Nay, 
why did I not abandon both plates, and simply cement my pile air¬ 
tight into the remote end of my tube ? All these devices passed 
through my mind, and formed subjects of experiment at an early 
stage of this inquiry. These experiments taught me that by 
bringing the gases into direct contact with my source of heat , or 
into direct contact with the face of my pile, I entirely vitiated 
my results. And this arrangement, which in my case would 
have been perfectly fatal as far as accuracy is concerned, is that 
which Professor Magnus has adopted, and is, I believe, the sole 
source of the differences which have shown themselves between 
his results and mine. 

His chief apparatus may be thus described:—A glass vessel 
fits like a receiver with its ground-edge on the plate of an air- 
pump. To the top of this receiver a second glass vessel is fused, 
and partially filled with water. Into this water steam is con¬ 
ducted, which causes the water to boil, a temperature of 100° C. 
being thus imparted to the bottom of the vessel, which is at the 
same time the top of the receiver. On the plate of the air- 
pump a thermo-electric plate is fixed with its face turned 
upwards, so as to receive the radiation from the heated top of 
the receiver. The face of the pile can be screened off at plea¬ 
sure from the radiation from above. Prom the pile, wires pro¬ 
ceed through the plate of the air-pump to the galvanometer. 
The receiver is first exhausted and the screen removed; the 
consequent deflection gives the amount of heat radiated against 
the pile through a vacuum. Air, or some other gas, is then 


416 


RECENT RESEARCHES ON RADIANT HEAT. 


admitted, and the reduction of the deflection is regarded as due 
to the absorption of the gas. 

Air at the common laboratory temperature is here admitted 
into direct contact with the radiating source of heat possessing a 
temperature of 100° C.; chilling of that source is the immediate 
consequence. And no matter how long the gas may remain 
there, the hot surface can never attain its pristine temperature. 
Professor Magnus, it will be observed, experiments in the 
ordinary way, making use of one face only of his pile. I entirely 
failed to obtain any absorption by air or any of the elementary 
gases by this mode of experiment, while he obtains for oxygen 
and air an absorption of 11 per cent., and for hydrogen an ab¬ 
sorption of 14 per cent. My apparatus enables me to measure 
an absorption of 0T per cent.; and surely with it an action so 
gross as the above could never have escaped me. Nor could it 
have escaped Melloni, who operated upon a column of air fifteen 
times the length of that used by Professor Magnus, and still 
found no absorption. With a column of air more than double 
the length of his I obtain for oxygen only x^th of the ab¬ 
sorption ascribed to it by Professor Magnus, and only T poth of 
what he finds for hydrogen. 

The greater action of hydrogen is quite in accordance with 
the known cliilling-power of that gas. While ascribing their 
results to a different cause, some experiments of my own, which 
are briefly described in the paper recently presented to the Poyal 
Society, completely corroborate those of Professor Magnus. In 
these experiments the gases were allowed to come into direct con¬ 
tact with the radiating source of heat, and here the action of 
hydrogen bore to that of oxygen the precise ratio found by 
Professor Magnus. The tube used in these experiments was 
8 inches long; and had I been tempted to ascribe the results to 
absorption, a tube of 8 inches would have yielded fifty times 
the effect observed in a tube of 33 inches, in which the gases 
were withdrawn from contact with the source of heat. 

The negative results of Professor Magnus, as regards aqueous 
vapour, are now sufficiently intelligible. The action which he 
observed in the case of air being due to direct chilling by con¬ 
tact—a process in which the mass of the chilling agent is the 
most important consideration—the action of the minute quan¬ 
tity of aqueous vapour present in the air becomes a vanishing 


RECENT RESEARCHES ON RADIANT HEAT. 


417 


quantity. He makes air more tlian a hundred times what 
it ought to be, and the action of the vapour practically dis¬ 
appears. 

It is curious and instructive to observe the contrast of opinion 
between Professor Magnus and myself. He concludes that, 
even if his experiments did not actually prove it, it must be 
evident that the small amount of aqueous vapour in the air 
cannot sensibly affect the absorption; and I apply the same con¬ 
sideration of smallness of quantity to account for the neutrality 
of the aqueous vapour, when mixed with air, as a chilling agent 
by contact. With regard to absorption, however, the quantity 
of vapour usually afloat in the atmosphere is large in com¬ 
parison with some of the quantities habitually employed in my 
experiments. 

Further, an inspection of these experiments showed me long 
ago that those substances which, in the liquid condition, are 
highly absorbent of radiant heat, are also highly absorbent in 

9 

the vaporous condition. How, water is proved by Melloni to be 
the most opaque liquid that he had examined; and it would be 
perfectly anomalous, on a priori grounds, if the vapour of this 
liquid proved so utterly neutral as the experiments of Professor 
Magnus make it. 

But the exposure of the naked face of the pile to the gas has 
also been spoken of. My experience of this arrangement is not 
without instruction. 

A square aperture was cut into a tin tube, and the face of a 
pile, cemented air-tight all round, introduced into the aperture. 
The tube was closed at the ends and connected with an air- 
pump. The tube being exhausted and the needle of the gal¬ 
vanometer connected with the pile at zero, -on allowing air to 
enter, the amount of heat generated dynamically and communi¬ 
cated to the pile, was sufficient to dash my needles against their 
stops at 90°. I do not entertain a doubt of being able to cause 
my needles to swing through an arc of 500° by the heat thus 
generated. When, on the contrary, the tube was full at the 
commencement, and the needle at zero, two or three strokes of 
the pump sufficed to send the needle up against the stops, the 
deflection now being due to the chilling of the thermo-pile. 
In fact this very deportment of a gaseous body on entering an 
exhausted receiver, and on being pumped out of a full one, has 
27 


418 


RECENT RESEARCHES ON RADIANT HEAT. 


enabled me to solve the paradoxical problem of determining the 
radiation and absorption of a gas or vapour without any source 
of heat external to the gaseous body itself. The pile of Pro¬ 
fessor Magnus was exposed to a similar action to that here described, 
though he never, to my knowledge, refers to it* It would be quite 
impossible for me to carry out my experiments with an instru¬ 
ment thus circumstanced; for after the pile had been either 
heated or chilled dynamically, it required in some cases hours 
for the needle to return to zero. I may add that these experi¬ 
ments on dynamic heating and chilling have been made with 
my needles loaded with pieces of paper, so as to render their 
motion visible to the most distant members of the large 
audience of the Royal Institution. 

In addition to the experiments made with the apparatus above 
described, Professor Magnus has made two other series with 
a glass tube one metre in length, stopped at its ends by plates 
of glass. His source of heat in this case was a powerful Argand 
lamp, the rays of which were collected by a parabolic mirror 
placed behind it. In one series the tube was covered within by 
a coating of blackened paper, while in the other this coating was 
removed, the radiation through the tube being augmented by the 
reflexion from its sides. With the blackened tube, Professor 
Magnus corroborates the results already obtained for air by Dr. 
Franz, who makes the absorption of a column of nearly the 
same length as that employed by Professor Magnus 8 per cent, 
of the incident heat. 

The difference between this result and that obtained with the 
other apparatus, which gave an absorption of 11 per cent., 
might naturally be ascribed to the different kinds of heat em¬ 
ployed in the two cases, were it not that in the series of experi¬ 
ments made with his unblackened tube, he finds the absorption 
of oxygen and of air to be 14*75 per cent.; and of hydrogen to 
be 16*23 per cent, of the incident heat. This great difference 
between the blackened and unblackened tube, Professor Mag¬ 
nus ascribes to a change of quality which the heat has undergone 
by reflexion at the interior surface of the tube, and which has 
rendered the heat more capable of absorption. I have tried to 
obtain this result with a glass tube of nearly the same length 


* See also Professor Wild’s Experiments, p. 389. 


RECENT RESEARCHES ON RADIANT HEAT. 


419 


as tliat used by Professor Magnus, but have failed to do so. 
The absorption of oxygen and air in his tube is 140 times , and the 
absorption of hydrogen is 160 times what they show themselves to 
be in mine. 

Whence these differences ? They are plainly to be referred 
to a source similar to that which caused the former ones. 
Indeed, I do not know a more instructive example of a single 
defect running through a long series of conscientious experi¬ 
ments, and so completely accounting for the observed anomalies. 
Professor Magnus stops his tube with plates of glass 4 milli¬ 
metres in thickness. Now Melloni has show that 61 per cent, 
of the rays of a Locatelli lamp are absorbed by a plate of glass 
2*6 millimetres in thickness. It is therefore almost certain that 
70 per cent, of the entire heat emitted by the lamp of Professor 
Magnus were lodged in his first glass plate. A much less 
quantity of the direct heat would be absorbed by his second 
plate; but here the amount absorbed would be most effective 
as a secondary source of heat, on account of the proximity of 
this plate to the thermo-electric pile. 

With the blackened tube, then, we had three sources of heat 
acting directly or indirectly upon the pile : the lamp, the first 
plate of glass, and the second plate. In reality, however, the 
sources of heat reduce themselves to two. Por, glass being 
opaque to the radiation from glass, the heat emitted by the first 
plate was expended in exalting the temperature of the second, 
close to which the pile was placed. On admitting air at the 
ordinary temperature into this tube, an effect similar in kind to 
that which takes place in the other instrument must occur : the 
heated glass plates are chilled, and they are chilled more by 
the hydrogen than by the air, thus giving us the exact results 
recorded by Professor Magnus. 

The same considerations applied to the unblackened tube 
explain perfectly the singular result obtained with it. On 
theoretic grounds it is extremely difficult, if not impossible, to 
conceive of such a change of quality in the heat as that above 
referred to. But there appears to be no reason to call in its 
aid. Professor Magnus himself finds that the quantity of heat 
transmitted through his unblackened tube is 26 times that 
which passes through his blackened one where the oblique 
radiation is cut off. In the*case therefore of the naked tube. 


420 


RECENT RESEARCHES ON RADIANT HEAT. 


the flux of heat sent down by the heated glass plate adjacent to 
the lamp, to its fellow at the other end, and likewise the heat 
sent directly from the lamp to the same plate, are greatly 
superior to what they are in the case of the blackened tube. 
The plate adjacent to the pile becomes therefore more highly 
heated ; and as its chilling is approximately proportionate to 
the difference of temperature between it and the cold air, the 
withdrawal of heat will be greatest when the tube is un¬ 
blackened within. While leaving myself open to correction, 

I would offer this as the explanation of the extraordinary result 
which Professor Magnus has obtained. It is, I submit, not a 
case of absorption, but of direct chilling by the cold air. 

It is hardly necessary to say that similar remarks to those 
made with reference to the blackened tube of Professor 
Magnus apply to the experiments of Dr. Franz. Dr. Franz 
never touched the absorption by air at all; his effects are 
entirely due to chilling by contact. This accounts for his 
finding the same effect in a tube 45 centimetres long as in a 
tube of 90 centimetres, for his ranking carbonic acid as low as 
air, while it is 90 times more powerful, and for making bro- . 
mine-vapour a greater absorbent than nitrous acid, whereas 
the absorption by the compound gas is vastly in excess. The 
heat rendered latent by the evaporation of his bromine, aug¬ 
mented the effect, which in reality he was measuring. In fact, 
all the differences between the German philosophers and myself 
appear to be strictly accounted for by reference to a source of 
error which the application of plates of rock-salt enabled me * 
from the outset to avoid. 


XIII. 


ON RADIATION THROUGH THE EARTH’S ATMOSPHERE* 

Nobody ever obtained the idea of a line from Euclid’s definition. The idea is 
obtained from a real physical line drawn by a pen or pencil, and therefore 
possessing width, the notion of width being afterwards dropped by a process of 
abstraction. So also with regard to physical phenomena : we conceive the in¬ 
visible by means of proper images derived from the visible, and purify our 
conceptions afterwards. Definiteness of conception, even though at some 
expense to delicacy, is of the greatest utility in dealing with physical phenomena. 
Indeed it may be questioned whether a mind trained in physical research can 
at all enjoy peace without having made clear to itself some possible way of 
imaging those operations which lie beyond the boundaries of sense, and in 
which sensible phenomena originate. 

It is well known that our atmosphere is mainly composed of the two elements 
oxygen and nitrogen. These elementary atoms may be figured as small spheres 
scattered thickly in the space which immediately surrounds the earth. They 
constitute about 99| per cent, of the atmosphere. Mixed with these atoms we 
have others of a totally different character; we have the molecules, or atomic 
groups, of carbonic acid, of ammonia, and of aqueous vapour. In these sub¬ 
stances diverse atoms have coalesced to form little systems of atoms. The 
molecules of aqueous vapour, for example, consist each of two atoms of hydrogen 
united to one of oxygen ; and they mingle as little triads among the monads of 
oxygen and nitrogen, which constitute the great mass of the atmosphere. 

A medium embraces our atoms; within our atmosphere exists a second and 
a finer atmosphere, in which the atoms of oxygen and nitrogen hang like 
suspended grains. This finer atmosphere unites not only atom with atom, but 
star with star; and the light of all suns, and of all stars, is in reality a kind of 
motion propagated through this interstellar medium. This image must be clearly 
seized, and then we have’to advance a step. We must not only figure our 
atoms suspended in this medium, but we must figure them vibrating in it. In 
this motion of the atoms consists what we call their heat. ‘What is heat in 
us,’ as Locke has perfectly expressed it, ‘ is in the body heated nothing but 
motion.’ We must figure this motion communicated to the medium in which , 
the atoms swing, and sent through it with inconceivable velocity. Motion in 
this form, unconnected with ordinary matter, but speeding through the inter¬ 
stellar medium, receives the name of Radiant Heat; and if competent to excite 
the nerves of vision, we call it Light. 

Aqueous vapour is an invisible gas. If vapour be permitted to issue horizon- 

* A public lecture, referred to at pp. 384 ancl 392 ; Proceedings of the Royal Institution 
vol. i*. p. 4. 


422 ON RADIATION THROUGH THE EARTH’S ATMOSPHERE. 


tally with considerable force from a tube connected with a small boiler, the 
track of the cloud produced by the precipitation of the vapour is seen. What is 
seen, however, is not vapour, but vapour condensed to water. Beyond the 
visible end of the jet the cloud resolves itself again into true vapour. A lamp 
placed under the jet cuts the cloud sharply off, and when the flame is placed 
near the efflux orifice the cloud entirely disappears. The heat of the lamp 
completely prevents precipitation. This same vapour may be condensed and 
congealed on the surface of a vessel containing a freezing mixture, from which 
it may be scraped in quantities sufficient to form a small snowball. When a 
luminous beam is sent through a large receiver placed on an air-pump, a single 
stroke of the pump causes the precipitation of the aqueous vapour to a cloud 
within. This, illuminated by the beam, produces upon a screen behind a richly- 
coloured halo, due to diffraction by the little cloud. 

The waves of heat pass from our earth through our atmosphere towards space. 
These waves meet in their passage the atoms of oxygen and nitrogen, and the 
molecules of aqueous vapour. Thinly scattered as these latter are, we might 
naturally think meanly of them as barriers to the waves of heat. We might 
imagine that the wide spaces between the vapour molecules would be an open 
door for the passage of the undulations; and that if those waves were at all 
intercepted, it would be by the substances which form 99£ per cent, of the 
whole atmosphere. It had, however, been found that this small modicum of 
aqueous vapour intercepts fifteen times the quantity of heat stopped by the 
whole of the air in which it was diffused. It was afterwards found that the 
dry air then experimented with was not perfectly pure, and that the purer the 
air became the more it approached the character of a vacuum, and the greater, 
by comparison, became the action of the aqueous vapour. The vapour was 
found to act with 30, 40, 50, 60, 70 times the energy of the air in which it was 
diffused ; and no doubt was entertained that the aqueous vapour of the air which 
filled the Royal Institution theatre, during the delivery of this discourse, quenched 
90 or 100 times the quantity of radiant heat absorbed by the main body of the 
air of the room. 

Looking at the single atoms, for every 200 of oxygen and nitrogen there is 
about 1 molecule of aqueous vapour. This 1, then, is 80 times more powerful than 
the 200 ; and hence, comparing a single atom of oxygen or nitrogen with a single 
molecule of aqueous vapour, we may infer that the action of the latter is 16,000 
times that of the former. This is a very astonishing result, and it naturally 
excited opposition, based on the philosophic reluctance to accept a fact of such 
import before testing it to the uttermost. From such opposition a discovery, 
if it be worth the name, emerges with its fibre strengthened; as the human 
character gathers force from the healthy antagonisms of active life. It was 
urged that the result was on the face of it improbable; that there were, more¬ 
over, many ways of accounting for it, without ascribing so enormous a compara¬ 
tive action to aqueous vapour. For example, the cylinder which contained the 
air in which these experiments were made, was stopped at its ends by plates 
of rock-salt, on account of their transparency to radiant heat. Now rock-salt is 
hygroscopic ; it attracts the moisture of the atmosphere. Thus, a layer of brine 
readily forms on the surface of a plate of rock-salt; and it is well known that 
brine is very impervious to the rays of heat. Breathing for a moment on a 
polished plate of rock-salt, the brilliant colours of thin plates (soap-bubble 
colours) flash forth, these being caused by the film of moisture which over- 


OX RADIATION THROUGH THE EARTH’S ATMOSPHERE. 423 


spreads the salt. Such a film, it was contended, is formed when undried air is 
sent.into the cylinder; it was, therefore, the absorption of a layer of brine that 
was measured, instead of the absorption of aqueous vapour. 

This objection was met in two ways :—First, by showing that the plates of 
salt when subjected to the strictest examination show no trace of a film of 
moisture. Secondly, by abolishing the plates of salt altogether, and obtaining 
the same results in a cylinder open at both ends. 

It was next surmised that the effect was due to the impurity of the laboratory 
air, and the suspended carbon particles were pointed to as the cause of the 
opacity to radiant heat. This objection was met by bringing air from Hyde 
Park, Hampstead Heath, Primrose Hill, Epsom Downs, afield near Newport in 
the Isle of Wight, St. Catharine’s Down, and the sea-beach near Black Gang 
Chine. The aqueous vapour of the air from these localities intercepted at 
least 70 times the amount of radiant heat absorbed by the air in which the 
vapour was diffused. Experiments made with dry smoky air proved that the 
atmosphere of West London, even when an east wind pours over it the smoke 
of the city, exerts only a fraction of the destructive powers exercised by the 
transparent and impalpable aqueous vapour diffused in the air. 

The cylinder which contained the air through which the calorific rays passed 
being polished within, the rays striking the interior surface were reflected from 
it to the thermo-electric pile. The following objection was raised You per¬ 
mit moist air to enter your cylinder ; a portion of this moisture is condensed as 
a liquid film upon the interior surface of your tube; its reflective power is 
thereby diminished; less heat therefore reaches the pile, and you incorrectly 
ascribe to the absorption of aqueous vapour an effect which is really due to 
diminished reflexion of the interior surface of your tube. 

But why should the aqueous vapour so condense? The tube within is 
warmer than the air without, and against its inner surface the rays of heat are 
impinging. There can be no tendency to condensation under such circumstances. * 
Further, let 5 inches of undried air be sent into the tube—that is, one-sixth 
of the amount which it can contain. These 5 inches produce their propor¬ 
tionate absorption. The driest day, on the driest portion of the earth’s surface, 
would make no approach to the dryness of our cylinder when it contains only 
5 inches of air. Make it 10, 15, 20, 25, 30 inches: you obtain an absorption 
exactly proportional to the quantity of vapour present. It is next to a physical 
impossibility that this could be the case if the effect were due to condensation. 
But lest a doubt should linger in the mind, not only were the plates of rock- 
salt abolished, but the cylinder itself was dispensed with. Humid air was 
displaced by dry, and dry air by humid in the free atmosphere; the absorption 
of the aqueous vapour was here manifest, as in all the other cases. 

No doubt, therefore, can exist of the extraordinary opacity of this substance 
to the rays of obscure heat; and particularly such rays as are emitted by the 
earth after it has been warmed by the sun. It is perfectly certain that more 
than 10 per cent, of the terrestrial radiation from the soil of England is stopped 
within 10 feet of the surface of the soil. This one fact is sufficient to show the 
immense influence which this newly-discovered property of aqueous vapour 

must exert on the phenomena of meteorology. 

This aqueous vapour is a blanket more necessary to the vegetable life of 

* This was saying too much. Professor Magnus has proved the existence of a kind of 
condensation under the conditions named. 


424 ON RADIATION THROUGH THE EARTH’S ATMOSPHERE. 


England than clothing is to man. Remove for a single summer-night the 
aqueous vapour from the air which overspreads this country, and you would 
assuredly destroy every plant capable of being destroyed by a freezing tempera¬ 
ture. The warmth of our fields and gardens would pour itself unrequited into 
space, and the sun would rise upon an island held fast in the iron grip of frost. 
The aqueous vapour constitutes a local dam, by which the temperature at the 
earth’s surface is deepened: the dam, however, finally overflows, and we give 
to space all that we receive from the sun. 

The sun raises the vapours of the equatorial ocean ; they rise, but for a time 
a vapour screen spreads above and around them. But the higher they rise, the 
more they come into the presence of pure space, and when, by their levity, they 
have penetrated the vapour screen, which lies clo%j to the earth’s surface, what 
must occur ? 

It has been said that, compared molecule with atom, the absorption of a 
molecule of aqueous vapour is 16,000 times that of air. Now the power to 
absorb and the power to radiate are perfectly reciprocal and proportional. 
The atom of aqueous vapour will therefore radiate with 16,000 times the energy 
of an atom of air. Imagine then this powerful radiant in the presence of space, 
and with no screen above it to check its radiation. Into space it pours its heat, 
chills itself, condenses, and the tropical torrents are the consequence. The 
expansion of the air, no doubt, also refrigerates it; but in accounting for those 
deluges, the chilling of the vapour by its own radiation must play a most im¬ 
portant part. The rain quits the ocean as vapour; it returns to it as water. 
How are the vast stores of heat set free by the change from the vaporous to the 
liquid condition disposed of P Doubtless in great part they are wasted by radia¬ 
tion into space. Similar remarks apply to the cumuli of our latitudes. The 
warmed air, charged with vapour, rises in columns, so as to penetrate the 
vapour screen which hugs the earth ; in the presence of space, the head of each 
pillar wastes its heat by radiation, condenses to a cumulus, which constitutes 
the visible capital of an invisible column of saturated air. 

Numberless other meteorological phenomena receive their solution, by 
reference to the radiant and absorbent properties of aqkeous vapour. It is 
the absence of this screen, and the consequent copious waste of heat, that 
causes mountains to be so much chilled when the sun is withdrawn. Its ab¬ 
sence in Central Asia renders the winter there almost unendurable; in Sahara 
the dryness of the air is sometimes such that, though during the day ‘ the soil 
is fire and the wind is flame,’ the chill at night is painful to bear. In 
Australia, also, the thermometric range is enormous, on account of the absence 
of this qualifying agent. A clear day, and a dry day, moreover, are very 
different things. The atmosphere may possess great visual clearness, while it 
is charged with aqueous vapour, and on such occasions great chilling cannot 
occur by terrestrial radiation. Sir John Leslie and others have been per¬ 
plexed by the varying indications of their instruments on days equally bright— 
but all these anomalies are completely accounted for by reference to this 
newly-discovered property of transparent aqueous vapour. Its presence would 
check the earth's loss; its absence, without sensibly altering the transparency 
of the air, would open wide a door for the escape of the earth’s heat into 
infinitude. 


XIY. 


OX A NEW SERIES OF CHEMICAL REACTIONS PRODUCED 

BY LIGHT. 

I 'wish to draw tlie attention of chemists to a form or method of experiment 
which, though obvious, is unknown, and which, I doubt not, will in their hands 
become a new experimental power. It consists in subjecting the vapours of 
volatile liquids to the action of concentrated sunlight, or to the concentrated 
beam of the electric-light. 

x 

Action of the Electric-light. 

A glass tube 2 - 8 feet long, and of 2-5 inches internal diameter, was supported 
horizontally. At one end of it was placed an electric lamp, the height and 
position of both being so arranged that the axis of the glass tube and that of 
the parallel beam issuing from the lamp were coincident. The tube in the first 
experiments was closed by plates of rock-salt, and subsequently by plates of 
glass. 

As on former occasions, for the sake of distinction, I will call this tube the 
experimental tube. 

The experimental tube was connected with an air-pump, and also with a 
series of drying and other tubes used for the purification of the air. 

A number of test-tubes (perhaps fifty in all) were converted into Woulfe’s 
flasks. Each of them was stopped with a cork, through which passed two glass 
tubes: one of these tubes («) ended immediately below the cork, while the 
other ( b ) descended to the bottom of the flask, being drawn out at its lower 
end to an orifice about 0-03 of an inch in diameter. It was found necessary 
to coat the cork carefully with cement. 

The little flask thus formed was partially filled with the liquid whose vapour 
was to be examined; it was then introduced into the path of a purified 
current of air. 

The experimental tube being exhausted, and the cock which admitted the 
purified air being cautiously turned on, the air entered the flask through the 
tube b , and escaped by the small orifice at the lower end of b into the liquid. 
Through this it bubbled, loading itself with vapour, after which the mixed air 
and vapour, passing from the flask by the tube «, entered the experimental tube, 
where they were subjected to the action of light. 

The power of the electric beam to reveal the existence of anything within 
the experimental tube, or the impurities of the tube itself, is extraordinary. 
When the experiment is made in a darkened room, a tube which in ordinary 


426 


CHEMICAL KEACTIONS PKODUCED BY LIGHT. 


daylight appears absolutely clean is often shown by the present mode of ex¬ 
amination to be exceedingly filthy. 

The following are some of the results obtained with this arrangement:— 

Nitrite of amyl (boiling-point 91° to 96° C.).—The vapour of this liquid was 
in the first instance permitted to enter the experimental tube while the beam 
from the electric lamp was passing through it. Curious clouds were observed 
to form near the place of entry, which were afterwards whirled through the 
tube. 

The tube being again exhausted, the mixed air and vapour were allowed to 
enter it in the dark. The slightly convergent beam of the electric-light was 
then sent through the tube from end to end. For a moment the tube was 
optically empty , nothing whatever was seen within it; but before a second had 
elapsed a shower of liquid spherules was precipitated on the beam, thus 
generating a cloud within the tube. This cloud became denser as the light 
continued to act, showing at some places a vivid iridescence. 

The beam of the electric lamp was now converged so as to form within the 
tube, between its end and the focus, a cone of rays about eight inches long. 
The tube was cleansed and again filled in darkness. When the light was sent 
through it, the precipitation upon the beam was so rapid and intense that the 
cone, which a moment before was invisible, flashed suddenly forth like a solid 
luminous spear. 

The effect was the same when the air and vapour were allowed to enter the 
tube in diffuse daylight. The cloud, however, which shone with such extra¬ 
ordinary radiance under the electric beam, was invisible in the ordinary light of 
the laboratory. 

The quantity of mixed air and vapour within the experimental tube could of 
course be regulated at pleasure. The rapidity of the action diminished with the 
attenuation of the vapour. When, for example, the mercurial column asso¬ 
ciated with the experimental tube was depressed only five inches, the action 
was not nearly so rapid as w r hen the tube was full. In such cases, however, it 
was exceedingly interesting to observe, after some seconds of waiting, a thin 
streamer of delicate bluish-white cloud slowly forming along the axis of the 
tube, and finally swelling so as to fill it. 

When dry oxygen was employed to carry in the vapour, the effect was the 
same as that obtained with air. 

When dry hydrogen was used as a vehicle, the effect was also the same. 

The effect, therefore, is not due to any interaction between the vapour of the 
nitrite and its vehicle. 

This was further demonstrated by the deportment of the vapour itself. 
When it was permitted to enter the experimental tube unmixed with air or any 
other gas, the effect was substantially the same. Hence the seat of the observed 
action is the vapour. 

With reference to the air and the glass of the experimental tube, the beam 
employed in these experiments was perfectly cold. It had been sifted by 
passing it through a solution of alum, and through the thick double-convex 
lens of the lamp. When the unsifted beam of the lamp was employed, the 
effect w r as still the same; the obscure calorific rays did not appear to interfere 
with the result. 

I have taken no means to determine strictly the character of the action here 
described, my object being simply to point out to chemists a method of experi- 


CHEMICAL REACTIONS PRODUCED BY LIGHT. 


427 


ment which reveals a new and beautiful series of reactions; to them I leave 
the examination of the products of decomposition. The molecule of the nitrite 
of amyl is shaken asunder by certain specific waves of the electric beam, 
forming nitric oxide and other products, of which the nitride of amyl is pro¬ 
bably one. The brown fumes of nitrous acid were seen to mingle with the 
cloud within the experimental tube. 

The nitrate of amyl, being less volatile than the nitrite, would not be able to 
maintain itself in the condition of vapour, but would be precipitated in liquid 
spherules along the track of the beam. 

In the anterior portions of the tube a sifting action of the vapour occurs 
which diminishes the chemical action in the posterior portions. In some 
experiments the precipitated cloud only extended halfway down the tube. 
When, under these circumstances, the lamp was shifted so as to send the beam 
through the other end of the tube, precipitation occurred there also. 

Action of Sunlight. 

The solar light also effects the decomposition of the nitrite-of-amyl vapour. 
On the 10th of October I partially darkened a small room in the Royal Institu¬ 
tion, into which the sun shone, permitting the light to enter through an open 
portion of the window-shutter. In the track of the beam was placed a large 
plano-convex lens, which formed a fine convergent cone in the dust of the 
room behind it. The experimental tube was filled in the laboratory, covered 
with a black cloth, and carried into the partially darkened room. On thrusting 
one end of the tube into the cone of rays behind the lens, precipitation within 
the cone was copious and immediate. The vapour at the distant end of the 
tube was in part shielded by that in front, and was also more feebly acted on 
through the divergence of tbe rays. On reversing the tube, a second and 
similar cone was precipitated. 

Physical Considerations. 

I sought to determine the particular portion of the white beam which pro¬ 
duced the foregoing effects. When, previous to entering the experimental tube, 
the beam was caused to pass through a red glass, the effect was greatly weak¬ 
ened, but not extinguished. This was also the case with various samples of 
yellow glass. A blue glass being introduced, before the removal of the yellow 
or the red, on taking the latter away augmented precipitation occurred along 
the track of the blue beam. Hence, in this case, the more refrangible rays are 
the most chemically active. 

The colour of the liquid nitrite of amyl indicates that this must be the case ; 
it is a feeble but distinct yellow: in other words, the yellow portion of the beam 
is most freely transmitted. It is not, however, the transmitted portion of any 
beam which produces chemical action, but the absorbed portion. Blue, as the 
complementary colour to yellow, is here absorbed, and hence the more energetic 
action of the blue rays. This reasoning, however, assumes that the same rays 
are absorbed by the liquid and its vapour. 

A solution of the yellow chromate of potash, the colour of which may be 
made almost, if not altogether, identical with that of the liquid nitrite of amyl, 
was found far more effective in stopping the chemical rays than either the red 


428 


CHEMICAL REACTIONS PRODUCED BY LIGHT. 


or the yellow glass. But of all substances the nitrite itself is most potent in 
arresting the rays which act upon its vapour. A layer one-eighth of an inch 
in thickness, which scarcely perceptibly affected the luminous intensity, 
sufficed to absorb the entire chemical energy of the concentrated beam of the 
electric-light. 

The close relation subsisting between a liquid and its vapour, as regards their 
action upon radiant heat, has been already amply demonstrated.* As regards 
the nitrite of amyl, this relation is more specific than in the cases hitherto 
adduced $ for here the special constituent of the beam which provokes the 
decomposition of the vapour is shown to be arrested by the liquid. 

A question of extreme importance in molecular physics here arises:—What 
is the real mechanism of this absorption, and where is its seat ? f 

I figure, as others do, a molecule as a group of atoms, held together by 
their mutual forces, but still capable of motion among themselves. The vapour 
of the nitrite of amyl is to be regarded as an assemblage of such molecules. 
‘The question now before us is this In the act of absorption, is it the mole¬ 
cules that are effective, or is it their constituent atoms ? Is the via viva of 
the intercepted waves transferred to the molecule as a whole, or to its consti¬ 
tuent parts ? 

The molecule, as a whole, can only- vibrate in virtue of the forces exerted 
between it and its neighbour molecules. The intensity of these forces, and 
consequently the rate of vibration, would, in this case, be a function of the 
distance between the molecules. Now the identical absorption of the liquid 
and of the vaporous nitrite of amyl indicates an identical vibrating period on 
the part of liquid and vapour, and this, to my mind, amounts to an experimental 
demonstration that the absorption occurs in the main within the molecule. For 
it can hardly be supposed, if the absorption were the act of the molecule as a 
whole, that it could continue to affect waves of the same period after the sub¬ 
stance had passed from the vaporous to the liquid state. 

In point of fact the decomposition of the nitrite of amyl is itself to some 
extent an illustration of this internal molecular absorption; for were the 
absorption the act of the molecule as a whole, the relative motions of its con¬ 
stituent atoms would remain unchanged, and there would be no mechanical 
cause for their separation. It is probably the synchronism of the vibrations of 
one portion of the molecule with the incident waves which enables the ampli¬ 
tude of those vibrations to augment until the chain which binds the parts of the 
molecule together is snapped asunder. 

The liquid nitrite of amyl is probably also decomposed by light; but the 
reaction, if it exists, is incomparably less rapid and distinct than that of the 
vapour. Nitrite of amyl has been subjected to the concentrated solar rays 
until it boiled, and it has been permitted to continue boiling for a considerable 
time, without any distinctly apparent change occurring in the liquid. 

I anticipate wide, if not entire, generality for the fact that a liquid and its 
vapour absorb the same rays. A cell of liquid chlorine now preparing for me 
will, I imagine, deprive light more effectually of its power of causing chlorine 
and hydrogen to combine than any other filter of the luminous ravs. The rays 
which give chlorine its colour have nothing to do with this combination, those 


* Philosophical Transactions, 1864 . 
f Mv attention was very forcibly directed to this subject 
ticni with my excellent friend Professor Clausius. 


some years ago by a conversa- 


CHEMICAL REACTIONS PRODUCED BY LIGHT. 


429 


v iiat are absorbed by the chlorine being the really effective rays. A highly 
sensitive bulb containing chlorine and hydrogen in the exact proportions neces¬ 
sary for the formation of hydrochloric acid was placed at one end of an experi¬ 
mental tube, the beam of the electric lamp being sent through it from the other. 
The bulb did not explode when the tube was tilled with chlorine, while the 
explosion was violent and immediate when the tube was filled with air. I 
anticipate for the liquid chlorine an action similar to but still more energetic 
than that exhibited by the gas. If this should prove to be the case, it will 
favour the view that chlorine itself is molecular and not monatomic. 

Production of the Blue of the Sky by the Decomposition of Nitrite of Amyl. 

When the quantity of nitrite-of-amyl vapour is considerable, and the light 
intense, the chemical action is exceedingly rapid, the particles precipitated being 
so large as to whiten the luminous beam. Not so, however, when a well-mixed 
and highly attenuated vapour fills the experimental tube. The effect now to 
be described was obtained in the greatest perfection when the vapour of the 
nitnite of amyl was derived from a residue of the moisture of its liquid, which 
had been accidentally introduced into the passage through which the dry air 
flowed into the experimental tube. 

In this case the electric beam traversed, the tube for several seconds before 
any action was visible. Decomposition then visibly commenced, and advanced 
slowly. The particles first precipitated were too small to be distinguished by 
an eye-glass; and, when the light was very strong, the cloud appeared of a 
milky blue. When, on the contrary, the intensity was moderate, the blue was 
pure and deep. In Briicke’s important experiments on the blue of the sky and 
the morning and evening red, pure mastic is dissolved in alcohol, and then 
dropped into water well stirred. When the proportion of mastic to alcohol is 
correct, the resin is precipitated so finely as to elude the highest microscopic 
power. By reflected light, such a medium appears bluish, by transmitted light 
yellowish, which latter colour, by augmenting the quantity of the precipitate, 
can be caused to pass into orange or red. 

But the development of colour in the attenuated nitrite-of-amyl vapour, 
though admitting of the same explanation, is doubtless more similar to what 
takes place in our atmosphere. The blue, moreover, is purer and more sky-like 
than that obtained from Briicke’s turbid medium. There could scarcely be a 
more impressive illustration of Newton’s mode of regarding the generation of 
the colour of the firmament than that here exhibited; for never, even in the 
skies of the Alps, have I seen a richer or a purer blue than that attainable by a ' 
suitable disposition of the light falling upon the precipitated vapour. May not 
the aqueous vapour of our atmosphere act in a similar manner ? and may we 
not fairly refer to liquid particles of infinitesimal size the hues observed by 
Principal Forbes over the safety-valve of a locomotive, and so skilfully con¬ 
nected by him with the colours of the sky ? 

In exhausting the tube containing the mixed air and nitrite-of-amyl vapour, 
it was difficult to avoid explosions under the pistons of the air-pump, similar to 
those described as occurring with the vapours of bisulphide of carbon and other 
substances.* Though the quantity of vapour present in these cases must have 
been infinitesimal, its explosion was sufficient to destroy the valves of the pump. 

* Page 28. 


430 


CHEMICAL REACTIONS PRODUCED BY LIGHT. 


Iodide of AlhjV (boiling-point 101° C.).—-Among the liquids hitherto sub¬ 
jected to the concentrated electric-light, iodide of allyl, in point of rapidity and 
intensity of action, comes next to the nitrite of amyl. With the iodide of 
allyl I have employed both oxygen and hydrogen, as well as air, as a vehicle, and 
found the effect in all cases substantially the same. The cloud column here 
was exquisitely beautiful, but its forms were different from those of the nitrite 
of amyl. The whole column revolved round the axis of the decomposing beam; 
it was nipped at certain places like an hour-glass, and round the two bells of 
the glass delicate cloud-filaments twisted themselves in spirals. It also folded 
itself into convolutions resembling those of shells. In certain conditions of the 
atmosphere in the Alps I have observed clouds of a special pearly lustre; 
when hydrogen was made the vehicle of the iodide-of-allyl vapour a similar 
lustre was most exquisitely shown. With a suitable disposition of the light, 
the purple hue of iodine vapour came out very strongly in the tube. 

The remark already made as to the bearing of the decomposition of nitrite of 
amyl by light on the question of molecular absorption applies here also; for 
were the absorption the work of the molecule as a whole, the iodine would not 
be dislodged from the allyl with which it is combined. The non-synchroftism 
of iodine with the waves of obscure heat is illustrated by its marvellous trans¬ 
parency to such heat. May not its synchronism with the waves of light in the 
present instance be the cause of its divorce from the allyl P Further experi¬ 
ments on this point are in preparation. 

Iodide of Isopropyl .—The action of light upon the vapour of this liquid is at 
first more languid than upon iodide of allyl; indeed many beautiful reactions 
may be overlooked in consequence of this languor at the commencement. 
After some minutes’ exposure, however, clouds begin to form, which grow in 
density and in beauty as the light continues to act. In every experiment 
hitherto made with'this substance the column of cloud which filled the experi¬ 
mental tube was divided into two distinct parts near the middle of the tube. 
In one experiment a globe of cloud formed at the centre, from which, right and 
left, issued an axis which united the globe with the two adjacent cylinders. 
Both globe and cylinders were animated by a common motion of rotation. As 
the action continued, paroxysms of motion were manifested; the various parts 
of the cloud would rush through each other with sudden violence. During 
these motions beautiful and grotesque cloud-forms were developed. At some 
places the nebulous mass would become ribbed, so as to resemble the graining 
of wood ; a longitudinal motion would at times generate in it a series of curved 
transverse bands, the retarding influence of the sides of the tube causing an 
appearance resembling, on a small scale, the dirt-bands of the Mer de Glace. 
In the anterior portion of the tube those sudden commotions were most intense ; 
here buds of cloud would sprout forth, and grow in a few seconds into perfect 
flower-like forms. 

A gorgeous mauve colour was developed in the last twelve inches of the 
tube; the vapour of iodine was present, and it may have been the sky-blue 
produced by the precipitated particles which, mingling with the purple of the 
iodine, produced this splendid mauve. As in all other cases here adduced, the 
effects were proved to be due to the light; they never occurred in darkness. 


XV. 


OX THE BLUE COLOUR OF THE SKY, THE POLARIZATION 
Oh* SKY-LIGHT, AND OX THE POLARIZATION OF LIGHT 
BY CLOUDY MATTER GENERALLY * 

Since the communication of my brief abstract ‘On a New Series of Chemical 
Reactions produced by Light/ the experiments upon this subject have been 
continued, and the number of the substances thus acted on considerably 
augmented. New relations have also been established between mixed vapours 
when subjected to the action of light. 

I now beg to draw attention to two questions glanced at incidentally in the 
abstract referred to—the blue colour of the sky, and the polarization of sky-light. 
Reserving the historic treatment of the subject for a more fitting occasion, I 
would merely mention now that these questions constitute, in the opinion of 
our most eminent authorities, the two great standing enigmas of meteorology. 
Indeed it was the interest manifested in them by Sir John Herschel, in a letter 
of singular speculative power, that caused me to enter upon the consideration 
of these questions so soon. 

The apparatus with which I work consists, as already stated, of a glass tube, 
about a yard in length, and from to 3 inches internal diameter. The vapour 
to be examined is introduced into this tube in the manner described in my last 
abstract, and upon it the condensed beam of the electric lamp is permitted to 
act until the neutrality or the activity of the substance has been declared. 

It has hitherto been my aim to render the chemical action of light upon 
vapours visible. For this purpose substances have been chosen, one at least of 
whose products of decomposition under light shall have a boiling-point so high 
that as soon as the substance is formed it shall be precipitated. By graduating 
the quantity of the vapour, this precipitation may be rendered of any degree of 
fineness, forming particles distinguishable by the naked eye, or particles which 
are probably far beyond the reach of our highest microscopic powers. 

I have no reason to doubt that particles may be thus obtained whose dia¬ 
meters constitute but a very small fraction of the length of a wave of violet 
light. 

In all cases when the vapours of the liquids employed are sufficiently attenu¬ 
ated, no matter what the liquid may be, the visible action commences with the 
formation of a blue cloud. I would guard myself at the outset against all 
misconception as to the use of this term. The blue cloud here referred to is 
totally invisible in ordinary daylight. To be seen, it requires to be surrounded 
by darkness, it only being illuminated by a powerful beam of light. This blue 

* From the Proceedings of the Royal Society , No. 108, 1869. 


432 


POLARIZATION OF LIGHT BY CLOUDY MATTER. 


cloud differs in many important particulars from the finest ordinary clouds, and 
might justly have assigned to it an intermediate position between these clouds 
and true cloudless vapour. 

With this explanation, the term 1 cloud/ or 1 incipient cloud/ as I propose to 
employ it, cannot he misunderstood. 

I had been endeavouring to decompose carbonic acid gas by light. A faint 
bluish cloud, due it may be, or it may not be, to the residue of some vapour 
previously employed, was formed in the experimental tube. On looking across 
this cloud through a Nicol’s prism, the line of vision being horizontal, it was 
found that when the short diagonal of the prism was vertical, the quantity of 
light reaching the eye was greater than when the long diagonal was vertical. 

When a plate of tourmaline was held between the eye and the bluish cloud, 
the quantity of light reaching the eye when the axis of the prism was perpen¬ 
dicular to the axis of the illuminating beam, was greater than when the axes 
of the crystal and of the beam were parallel to each other. 

This was the result all round the experimental tube. Causing the crystal of 
tourmaline to revolve round the tube, with its axis perpendicular to the illumina¬ 
ting beam, the quantity of light that reached the eye was in all its positions a 
maximum. When the crystallographic axis was parallel to the axis of the beam, 
the quantity of light transmitted by the crystal was a minimum. 

From the illuminated bluish cloud, therefore, polarized light was discharged, 
the direction of maximum polarization being at right angles to the illuminating 
beam ; the plane of vibration of the polarized light, moreover, was that to which 
the beam was perpendicular.* 

Thin plates of selenite or of quartz, placed between the Nicol and the bluish 
cloud, displayed the colours of polarized light, these colours being most vivid 
when the line of vision was at right angles to the experimental tube. The 
plate of selenite usually employed was a circle, thinnest at the centre, and 
augmenting uniformly in thickness from the centre outwards. When placed in 
its proper position between the Nicol and the cloud, it exhibited a system of 
splendidly coloured rings. 

The cloud here referred to was the first operated upon in the manner described. 
It may, however, be greatly improved upon by the choice of proper substances, 
and by the application in proper quantities of the substances chosen. Benzol, 
bisulphide of carbon, nitrite of amyl, nitrite of butyl, iodide of allyl, iodide of 
isopropyl, and many other substances may be employed. I will take the nitrite 

b \ as 11 istiative of the means adopted to secure the best result with 
reference to the present question. 

And here it may be mentioned that a vapour, which when alone, or mixed 
with air in the experimental tube, resists the action of light, or shows but a 
feeble result of this action, may, by placing it in proximity with another gas or 
vapour, be caused to exhibit imder light vigorous, if not violent, action. The 
case is similar to that of carbonic acid gas, which diffused in the atmosphere 
resists the decomposing action of solar light, but when placed in contiguity 
with the chlorophyl in the leaves of plants, has its molecules shaken asunder. 

* I assume here that the plane of vibration is perpendicular to the plane of polarization. 
This is still an undecided point; but the probabilities are so much in its favour, and it is 
in my opinion so much preferable to have a physical image on which the mind can rest 
that I do not hesitate to employ the phraseology in the text. Even should the assumption 
prove to be incorrect, no harm will be done by the provisional use of it. 


POLARIZATION OF LIGHT BY CLOUDY MATTER. 433 


Dry air was permitted to bubble through the liquid nitrite of butyl until the 
experimental tube, which had been previously exhausted, was filled with the 
mixed air and vapour. The visible action of light upon the mixture after 
fifteen minutes’ exposure was slight. The tube was afterwards filled with half 
an atmosphere of the mixed air and vapour, and another half atmosphere of air 
which had been permitted to bubble through fresh commercial hydrochloric 
acid. On sending the beam through this mixture, the action paused barely 
sufficiently long to show that at the moment of commencement the tube was 
optically empty. But the pause amounted only to a small fraction of a second, 
a dense cloud being immediately precipitated upon the beam which traversed 
the mixture. 

This cloud began blue , but the advance to whiteness was so rapid as almost 
to justify the application of the term instantaneous. The dense cloud, looked 
at perpendicularly to its axis, showed scarcely any signs of polarization. Looked 
at obliquely the polarization was strong. 

The experimental tube being again cleansed and exhausted, the mixture of air 
and nitrite-of-butyl vapour was permitted to enter it until the associated 
mercury column was depressed of an inch. In other words, the air and 
vapour, united, exercised a pressure not exceeding ^ of an atmosphere. Air 
passed through a solution of hydrochloric acid was then added till the mercury 
column was depressed three inches. The condensed beam of the electric-light 
passed for some time in darkness through this mixture. There was absolutely 
nothing within the tube competent to scatter the light. Soon, however, a 
superbly blue cloud was formed along the track of the beam, and it continued 
blue sufficiently long to permit of its thorough examination. The light dis¬ 
charged from the cloud at right angles to its own length was perfectly polarized. 
% degrees the cloud became of whitish-blue, and for a time the selenite 
colours obtained by looking at it normally were exceedingly brilliant. The 
direction of maximum polarization was distinctly at right angles to the illumi¬ 
nating beam. This continued to be the case as long as the cloud maintained a 
decided blue colour, and even for some time after the pure blue had changed to 
whitish-blue. But as the light continued to act the cloud became coarser and 
whiter, particularly at its centre, where it at length ceased to discharge polarized 
light in the direction of the perpendicular, while it continued to do so at both 
its ends. 

But the cloud which had thus ceased to polarize the light emitted normally, 
showed vivid selenite colours when looked at obliquely. The direction of 
maximum polarization changed with the texture of the cloud. This point shall 
receive further illustration subsequently. 

A blue, equally rich and more durable, was obtained by employing the 
nitrite-of-butyl vapour in a still more attenuated condition. Now the instance 
here cited is representative. In all cases, and with all substances, the cloud 
formed at the commencement, when the precipitated particles are sufficiently 
fine, is blue , and it can be made to display a colour rivalling that of the purest 
Italian sky. In all cases, moreover, this fine blue cloud polarizes perfectly the 
beam which illuminates it, the direction of polarization enclosing an angle of 
90° with the axis of the illuminating beam. 

It is exceedingly interesting to observe both the perfection and the decay of 
this polarization. For ten or fifteen minutes after its first appearance the light 
28 


434 POLARIZATION OF LIGHT BY CLOUDY MATTER. 


from a vividly illuminated incipient cloud, looked at horizontally, is absolutely 
quenched by a Nicol’s prism with its longer diagonal vertical. But as the sky- 
blue is gradually rendered impure by the introduction of particles of too large a 
size, in other words, as real clouds begin to be formed, the polarization begins 
to deteriorate, a portion of the light passing through the prism in all its posi¬ 
tions. It is worthy of note that for some time after the cessation of perfect 
polarization the residual light which parses, when the Nicol is in its position of 
minimum transmission, is of a gorgeous blue, the whiter light of the cloud 
being extinguished.* When the cloud texture has become sufficiently coarse 
to approximate to that of ordinary clouds, the rotation of the Nicol ceases to 
have any sensible effect on the quality of the light discharged normally. 

The perfection of the polarization in a direction perpendicular to the illumi¬ 
nating beam is also illustrated by the following experiment. A Nicol’s prism 
large enough to embrace the entire beam of the electric lamp was placed 
between the lamp and the experimental tube. A few bubbles of air carried 
through the liquid nitrite of butyl were introduced into the tube, and they were 
followed by about 3 inches (measured by the mercurial gauge) of air which 
had been passed through aqueous hydrochloric acid. Sending the polarized 
beam through the tube, I placed myself in front of it, my eye being on a level 
with its axis, my assistant, Mr. Cottrell, occupying a similar position behind 
the tube. The short diagonal of the large Nicol was in the first instance 
vertical, the plane of vibration of the emergent beam being therefore also 
vertical. As the light continued to act, a superb blue cloud, visible to both my 
assistant and myself, was slowly formed. But this cloud, so deep and rich when 
looked at from the positions mentioned, utterly disappeared when looked at verti¬ 
cally downwards^ or vertically upwards. Reflexion from the cloud was not 
possible in these directions. When the large Nicol was slowly turned round its 
axis, the eye of the observer being on the level of the beam, and the line of 
vision perpendicular to it, entire extinction of the light emitted horizontally 
occurred where the longer diagonal of the large Nicol was vertical. But now 
a vivid blue cloud was seen when looked at downwards or upwards. This truly 
fine experiment was first definitely suggested by a remark addressed to me in a 
letter by Professor Stokes. 

Now, as regards the polarization of sky-light, the greatest stumblingblock has 
hitherto been that, in accordance with the law of Brewster, which makes the 
index of refraction the tangent of the polarizing angle, the reflexion which pro¬ 
duces perfect polarization would require to be made in air upon air; and indeed 
this led many of our most eminent men, Brewster himself among the 
number, to entertain the idea of molecular reflexion. I have, however, operated 
upon substances of widely different refractive indices, and therefore of very 
different polarizing angles as ordinarily defined, but the polarization of the 
beam by the incipient cloud has thus far proved itself to be absolutely indepen¬ 
dent of the polarizing angle. The law of Brewster does not apply to matter in 
this condition, and it rests with the undulatory theory to explain why. When¬ 
ever the precipitated particles are sufficiently fine, no matter what the substance 
forming the particles may be, the direction of maximum polarization is at 

* This seems to prove that particles too large to polarize the blue, polarize perfectly li^bt 
of lower refrangibility. 


POLARIZATION OF LIGHT BY CLOUDY MATTER. 435 

right angles to the illuminating beam, the polarizing angle for matter in this 
condition being invariably 45°.* 

. Suppose our atmosphere surrounded by an envelope impervious to light, but 
with an aperture on the sunward side through which a parallel beam of solar 
light could enter and traverse the atmosphere. Surrounded on all sides by air 
not directly illuminated, the track of such a beam through the air would re¬ 
semble that of the parallel beam of the electric lamp through an incipient 
cloud. The sunbeam would be blue, and it would discharge laterally light in 
precisely the same condition as that discharged by the incipient cloud. In fact, 
the azure revealed by such a beam would be jto all intents and purposes that 
which I have called a ‘blue cloud.’ 

But, as regards the polarization of the sky, we know that not only is the 
direction of maximum polarization at right angles to the track of the solar 
beams, but that at certain angular distances, probably variable ones, from the 
sun, ‘ neutral points,’ or points of no polarization, exist, on both sides of which 
the planes of atmospheric polarization are at right angles to each other. 

The parallel beam employed in these experiments tracked its way through 
the laboratory air exactly as sunbeams are seen to do in the dusty air of London. 
This air showed, though far less vividly, all the effects of polarization obtained 
with the incipient clouds. The light discharged laterally from the track of the 
illuminating beam was polarized, though not perfectly, the direction of maxi¬ 
mum polarization being at right angles to the J)eam. 

The horizontal column of air thits illuminated was 18 feet long, and could 
therefore be looked at very obliquely without any disturbance from a solid 
envelope. At all points of the beam throughout its entire length the light 
emitted normally was in the same state of polarization. Keeping the positions 
of the Nicol and the selenite constant, the same colours were observed 
throughout the entire beam when the line of vision was perpendicular to its 
length. 

* The difficulty referred to above is thus expressed by Sir John Herschel:—‘The cause 
of the polarization is evidently a reflexion of the sun’s light upon something. The question 
is on what ? Were the angle of maximum polarization 76°, we should look to water or 
ice as the reflecting body, however inconceivable the existence in a cloudless atmosphere, 
and a hot summer’s day of unevaporated molecules (? particles) of water. But though we 
were once of this opinion, careful observation has satisfied us that 90°, or thereabouts, is a 
correct angle, and that therefore, whatever be the body on which the light has been 
reflected, if polarized by a single reflexion, the polarizing angle must be 45°, and the index 
of refraction, which is the tangent of that angle, unity ; in other words, the reflexion 
would require to be made in air upon air! ’ ( Meteorology , par. 233). 

Anv particles, if small enough, will produce both the colour and the polarization of the 
sky. * But is the existence of small water-particles on a hot summer’s day in the higher 
regions of our atmosphere inconceivable? It is to be remembered that the oxygen and 
nitrogen of the air behave as a vacuum to radiant heat, the exceedingly attenuated vapour 
of the higher atmosphere being therefore in practical contact with the cold of space. 

The opinion of Sir John Herschel, connecting the polarization and the blue colour of 
the sky is verified by the foregoing results. ‘The more the subject [the polarization of 
skv-light] is considered,’ writes this eminent philosopher, ‘ the more it will be found beset 
with difficulties, and its explanation when arrived at will probably be found to carry with 
it that of the blue colour of the sky itself and of the great quantity of light it actually does 
send down to us.’ * We may observe, too,’ he adds, ‘that it is only where the purity of the 
skv is most absolute that the polarization is developed in its highest degree, and that where 
there is the slightest perceptible tendency to cirrus it is materially impaired.’ This applies 
word for word to the ‘ incipient clouds.’ # 


436 POLARIZATION OF LIGHT BY CLOUDY MATTER. 

X tlien placed myself near the end of the beam as it issued from the electric 
lamp, and looking through the Nicol and selenite more and more obliquely at 
the beam, observed the colours fading until they disappeared. Augmenting the 
obliquity, the colours appeared once more, but they were now complementary to 
the former ones. 

Ilence this beam, like the sky, exhibited its neutral point, at opposite sides 
of which the light was polarized in planes at right angles to each other. 

Thinking that the action observed in the laboratory might be caused in some 
way by the vaporous fumes diffused in its air, I had a battery and an electric 
lamp carried to a room at the tqp of the Royal Institution. The track of the 
beam was seen very finely in the air of this room, a length of 14 or 15 feet 
being attainable. This beam exhibited all the effects observed with the beam 
in the laboratory. Even the uncondensed electric-light falling on the floating 
matter showed, though faintly, the effects of polarization.* 

AVhen the air was so sifted as to entirely remove the visible floating 
matter, it no longer exerted any sensible action upon the light, but behaved like 
a vacuum. 

I had varied and confirmed in many ways those experiments on neutral points, 
^operating upon the fumes ot chloride of ammonium, the smoke of brown paper, 
and tobacco smoke, when my attention was drawn by Sir Charles Wheatstone 
to an important observation communicated to the Paris Academy in 1860 by 
Professor Govi, of Turin. His.observations on the light of comets had led 
M. Govi to examine a beam of light seht through a room in which was 
diffused the smoke of incense. lie also operated on tobacco smoke. Ilis first 
biiet communication stated the fact of polarization by such smoke, but in his 
second communication he announced the discovery of a neutral point in the 
beam, at the opposite sides of which the light was polarized in planes at right 
angles to each other. 

But, unlike my observations on the laboratory air, and unlike the action of 
the sky, the direction of maximum polarization in M. Govi’s experiments en¬ 
closed a very small angle with the axis of the illuminating beam. The question 
was left in this condition, and I am not aware that JM. Govi or any other inves¬ 
tigator has pursued it farther. 

I had noticed, as before stated, that as the clouds formed in the experimental 
tube became denser, the polarization of the light discharged at right angles to 
the beam became weaker, the direction of maximum polarization becoming 
oblique to the beam. Experiments on the fumes of chloride of ammonium gave 
me also reason to suspect that the position of the neutral point was not constant } 
but that it varied with the density of the illuminating fumes. 

X he examination of these questions led to the following new and remarkable 
results.—The laboratory being well filled with the fumes of incense, and 
sufficient time bein^ allowed for their uniform diffusion, the electric beam 
was sent thiough the smoke. From the track ot the beam polarized light 
'vsas discharged, but the direction of maximum polarization, instead of being 
along the normal, now enclosed an angle of 12° or 13° with the axis of the 
beam. 

A neutral point, with complementary effects at opposite sides of it, was also 
exhibited by the beam. Ihe angle enclosed by the axis of the beam, and a 

* I hope to try Alpine air next summer, 
f Comptes Rendusy tome li. pp. 360 and 669. 


POLARIZATION OF LIGHT BY CLOUDY MATTER. 437 

line drawn from the neutral point to the observer’s eye, measured in the first 
instance 06°. 

The windows of the laboratory were now opened for some minutes, a portion 
of the incense smoke being permitted to escape. On again darkening the room 
and turning on the beam, the line of vision to the neutral point was found to 
enclose with the axis of the beam an angle of 68°. 

The windows were again opened for a few minutes, more of the smoke being 

permitted to escape. Measured as before the angle referred to was found to 
be 54°. 

This process was repeated three additional times; the neutral point was found 
to recede lower and lower down the beam, the angle between a line drawn 
from the eye to the neutral point and the axis of the beam falling successively 
from 54°, to 49°, 43°, and 33°. 

The distances, roughly measured, of the neutral point from the lamp, corre¬ 
sponding to the foregoing series of observations, were these:— 


1st observation 2 feet 2 inches. 


2nd 

99 

2 

99 

6 

99 

3rd . 

99 

2 

99 

10 

99 

4th 

99 

3 

99 

2 

99 

5th 

99 

3 

99 

7 

99 

6th 

99 

4 

99 

6 

99 


At the end of this series of experiments the direction of maximum polari¬ 
zation had again become normal to the beam. 

The laboratory was next tilled with the fumes of gunpowder. In five suc¬ 
cessive experiments, corresponding to five different densities of the gunpowder 
smoke, the angles enclosed between the line of vision to the neutral point and 
the axis of the beam were 63°, 50°, 47°, 42°, and 38° respectively. 

After the clouds of gunpowder had cleared away, the laboratory was tilled 
with the fumes of common resin, rendered so dense as to be very irritating to 
the lungs. The direction of maximum polarization enclosed in this case an 
angle of 12°, or thereabouts, with the axis of the beam. Looked at, as in the 
former instances, from a position near the electric lamp no neutral point was 
observed throughout the entire extent of the beam. 

When this beam was looked at normally through the selenite and Nicol, the 
ring system, though not brilliant, was distinct. Keeping the eye upon the 
plate of selenite and the line of vision normal, the windows were opened, the 
blinds remaining undrawn. The resinous fumes slowly diminished, and as 
they did so the ring system became paler. It finally disappeared. Continuing 
to look along the perpendicular, the rings revived, but now the colours 
were complementary to the former ones. The neutral point had passed me 
in its motion down the beam consequent upon the attenuation of the fumes of 
resin. 

In the fumes of chloride of ammonium substantially the same results were 
obtained as those just described. Sufficient, I think, has been here stated to 
illustrate the variability of the position of the neutral point.* 

* Brewster has proved the variability of the position of the neutral point for sky-light 
with the sun’s altitude. Is not the proximate cause of this revealed by the foregoing 
experiments V 



438 POLARIZATION OF LIGHT BY CLOUDY MATTER. 


Before quitting the question of the reversal of the polarization by cloudy 
matter, I will make one or two additional observations. Some of the clouds 
formed in the experiments on the chemical action of light are astonishing as to 
form. The experimental tube is often divided into segments of dense cloud, 
separated from each other by nodes of finer matter. Looked at normally, as 
many as four reversals of the plane of polarization have been found in the tube 
in passing from node to segment, and from segment to node. With the fumes 
diffused in the laboratory, on the contrary, there was no change in the polariza¬ 
tion along the normal, for here the necessary differences of cloud texture did 
not exist. 

Further. By a puff of tobacco smoke or of condensed steam blown into the 
illuminated beam, the brilliancy of the colours may be greatly augmented. But 
with different clouds two different effects are produced. For example, let the 
ring system observed in the common air be brought to its maximum strength, 
and then let an attenuated cloud of chloride of ammonium be thrown into the 
beam at the point looked at: the ring system flashes out with augmented 
brilliancy, and the character of the polarization remains unchanged. This is 
also the case when phosphorus or sulphur is burned underneath the beam, so as 
to cause the fine particles of phosphoric acid or of sulphur to rise into the 
light. ~V\ ith the sulphur-fumes the brilliancy of the colours is exceedingly 
intensified j but in none of these cases is there any change in the character of 
the polarization. 

But when a puff of aqueous cloud, or of the fumes of hydrochloric acid, 
hjdiiodic acid, or nitric acid is thrown into the beam, there is a complete 
reversal of the selenite tints. Each of these clouds twists the plane of polari¬ 
zation 90°. On these and kindred points experiments are still in progress.* 

The idea that the colour of the sky is due to the action of finely divided 
matter, rendering the atmosphere a turbid medium, through which .we look at 
the darkness of space, dates as far back as Leonardo da Y inci. Newton con¬ 
ceived the colour to be due to exceedingly small water particles acting as thin 
plates. Goethe’s experiments in connexion with this subject are well known 
and exceedingly instructive. One very -striking observation of Goethe’s referred 
to what is technically called ‘chill’ by painters, which is due no doubt to 
extremely fine varnish particles interposed between the eye and a dark back¬ 
ground. Clausius, in two very able memoirs, endeavoured to connect the 
colours of the sky with suspended water-vesicles, and to show that the impor¬ 
tant observations ot Forbes on condensing steam could also be thus accounted 
for. Briicke’s experiments on precipitated mastic were referred to in my last 
abstract. Helmholtz has ascribed the blueness of the eyes to the action of 
suspended particles. In an article written nearly nine years ago by myself, the 
colours of the peat smoke of the cabins of Killamey f and the colours of the 
sky were referred to one and the same cause,* while a chapter of the * Glaciers 
of the Alps,’ published in 18G0, is also devoted to this question. Roscoe, in 


. Sir John Berschel has suggested to me that this change of the polarization from posi. 
tive to negative may indicate a change from polarization by reflexion to polarization by 
re raction. This thought repeatedly occurred to me while iooking at the effects; but it 
will require much following up before it emerges into clearness. 

t I have sometimes quenched almost completely, by a Nicol, the light discharged 
normally from burning leaves in Hyde Park. The blue smoke from the ignited end of a 
cigar polarizes also, but not perfectly. 


POLARIZATION OF LIGHT BY CLOUDY MATTER. 439 


connexion with his truly beautiful experiments on the photographic power of 
sky-light, has also given various instances of the production of colour by 
suspended particles. In the foregoing experiments the azure was produced in 
air, and exhibited a depth and purity far surpassing anything that I have ever 
seen in mote-filled liquids. Its polarization, moreover, was 'perfect. 

In his experiments on fluorescence Professor Stokes had continually to 
separate the light reflected from the motes suspended in his liquids, the action 
of which he named i false dispersion/ from the fluorescent light of the same 
liquids, which he ascribed to ‘ true dispersion.’ In fact, it is hardly possible to 
obtain a liquid without motes, which polarize by reflexion the light falling 
upon them, truly dispersed light being unpolarized. At p. 530 of his celebrated 
memoir ‘ On the Change of the Itefrangibility of Light,’ Professor Stokes adduces 
some significant facts, and makes some noteworthy remarks, which bear upon 
our present subject. He notices more particularly a specimen of plate-glass 
which, seen by reflected light, exhibited a blue which was exceedingly like an 
effect of fluorescence, but which, when properly examined, was found to be an 
instance of false dispersion. 1 It often struck me,’ he writes, 1 while engaged in 
these observations, that when the beam had a continuous appearance, the 
polarization was more nearly perfect than when it was sparkling, so as to force 
on the mind the conviction that it arose merely from motes.* Indeed in the 
former case the polarization has often appeared perfect, or all but perfect. It 
is possible that this may in some measure have been due to the circumstance, 
that when a given quantity of light is diminished in a given ratio, the illumi¬ 
nation is perceived with more difficulty when the light is diffused uniformly 
than when it is spread over the same space, but collected into specks. Be 
this as it may, there was at least no tendency observed towards polarization in 
a plane perpendicular to the plane of reflexion, when the suspending particles 
became finer, and therefore the beam more nearly continuous.’ 

Through the courtesy of its owner, I have been permitted to see and to 
experiment with the piece of plate-glass above referred to. Placed in front 
of the electric lamp, whether edgeways or transversely, it discharges bluish 
polarized light laterally, the colour being by no means a bad imitation of the 
blue of the sky. 

Professor Stokes considers that this deportment may be invoked to decide 
the question of the direction of the vibrations of polarized light. On this point 
I would say, if it can be demonstrated that when the particles are small in 
comparison to the length of a wave of light, the vibrations of a ray reflected by 
such particles cannot be perpendicular to the vibrations of the incident light; 
then assuredly the experiments.recorded in the foregoing communication decide 
the question in favour of Fresnel’s assumption. 

As stated above, almost all liquids have motes in them sufficiently numerous 
to polarize sensibly the light, and very beautiful effects may be obtained by 
simple artificial devices. When, for example, a cell of distilled water is placed 
in front of the electric lamp, and a slice of the beam permitted to pass through 
it, scarcely any polarized light is discharged, and scarcely any colour produced 

* The azure may be produced in the midst of a field of motes. By turning the Nicol, 
the interstitial blue may be completely quenched, the shining, and apparently unaffected? 
motes remaining masters of the field. A blue cloud, moreover, may be precipitated in the 
midst of the azure. An aqueous cloud thus precipitated reverses the polarization; but on 
the melting away of the cloud the azure and its polarization remain behind. 


440 POLARIZATION OF 


LIGHT BY CLOUDY MATTER. 


with a plate of selenite. But while the beam is passing through it, if a bit of 
soap be agitated in the water above the beam, the moment the infinitesimal 
particles reach the beam the liquid sends forth laterally almost perfectly 
polarized light; and if the selenite be employed, vivid colours flash into exist¬ 
ence. A still more brilliant result is obtained with mastic dissolved in a great 
excess of alcohol. 

The selenite rings constitute an extremely delicate test as to the quantity of 
motes m a liquid. Commencing with distilled water, for example, a thickish 
beam of light is necessary to make the polarization of its motes sensible. A 
much thinner beam suffices for common water j while with Briicke’s precipi¬ 
tated mastic, a beam too thin to produce any sensible effect with most other 
liquids, suffices to bring out vividly the selenite colours. 







j 


XVI. 


OX COMETARY THEORY* 

On the 8th of May 1869, in a lecture delivered before the Cambridge Philo¬ 
sophical Society, I ventured to enunciate a speculation regarding the origin and 
deportment of visible cometary matter. I had been led to reflect on the subject 
by my experiments on the decomposition of vapours by light. The speculation 
was introduced and communicated to the Philosophical Society in the following 
words:— 

1 In the course of my experiments on actinic action I have been often astonished 
at the body of light which a perfectly infinitesimal amount of matter, when 
diffused in the form of a cloud, can discharge from it by reflexion. I have been 
repeatedly perplexed and led into error by the action of residues so minute as 
to be simply inconceivable. In order to get rid of these residues, my experi¬ 
mental tubes, after having been employed for any vapour, are flooded with 
alcohol, sponged-out with soap and hot water, and finally flooded with pure 
water. Let me give you some idea of the quantities of matter that here come 
into play. The tube before you, which is 3 feet long and 3 inches wide, was 
so thoroughly cleansed that when filled with air, or with the vapour of aqueous 
hydrochloric acid, no amount of exposure to an intense light produced the least 
cloudiness. Having thus assured myself of the perfect purity of the tube, I 
took a small bit of bibulous paper, rolled up into a pellet not the fourth part of 
the size of a small pea, and moistened it with a liquid possessing a higher 
boiling-point than that of water. I held the pellet with my fingers until it had 
become almost dry, then introduced it into a connecting-piece, and allowed dry 
air to pass over it into this tube. The air charged with the modicum of vapour 
thus taken up was subjected to the action of light. A blue actinic cloud began 
to form immediately, and in five minutes the blue colour had extended quite 
through the Experimental tube. For some minutes this cloud continued blue, 
and could be completely quenched by a Nicol’s prism, no trace of its light reach¬ 
ing the eye when the Nicol was in its proper position. But its particles 
augmented gradually in magnitude, and at the end of fifteen minutes a dense 
white cloud filled the tube. Considering the amount of the vapour carried in 
by the air, the appearance of a cloud so massive and luminous seemed like the 
creation of a world out of nothing. 

* But this is not all; the pellet of bibulous paper was removed, and the 
experimental tube was cleared out by sweeping a current of dry air through it. 
This current passed also through the connecting-piece in which the pellet of bibulous 
paper had rested. The air was at length cut otf and the experimental tube 

* Philosophical Magazine for April 1869. 


442 


ON COMETARY THEORY. 


exhausted. Fifteen inches of hydrochloric acid were then sent into the tube 
through the same connecting-piece. Now it is here to be noted : (1) that the 
total quantity of liquid absorbed by the pellet in the first instance was exceed¬ 
ingly small; (2) that nearly the whole of this small quantity had been allowed 
to evaporate between my fingers before the pellet was placed in the connecting- 
piece ; (3) that the pellet had been ejected and the tube in which it rested 
rendered for some minutes the conduit of a strong current of pure air. It was 
part of such a residue as could linger in the connecting-piece after this process, 
that was carried into the experimental tube by the hydrochloric acid and sub¬ 
jected there to the action of light. 

‘One minute after the ignition of the electric lamp a faint cloud showed 
itself; in two minutes it had filled all the anterior portion of the tube and 
stretched a considerable way down it; it developed itself afterwards into a very 
beautiful cloud-figure ; and at the end of fifteen minutes the body of light dis¬ 
charged by the cloud, considering the amount of matter involved in its produc¬ 
tion, was simply astounding. But though thus luminous, the cloud was far too 
fine to dim in any appreciable degree objects placed behind it. The flame of a 
candle seemed no more affected by it than it would be by. a vacuum. Placing 
a page of print so that it might be illuminated by the cloud itself, it could be 
read through the cloud without any sensible enfeeblement. Nothing could 
more perfectly illustrate that ‘ spiritual texture ’ which Sir John Herschel 
ascribes to a comet than these actinic clouds. Indeed, experiments proved that 
matter of almost infinite tenuity is competent to shed forth light far more 
intense than that of the tails of comets. The weight of the matter which 
Sent this body of light to the eye, would probably have to be multiplied by 
millions to bring it up to the weight of the air in which it hung. 

1 And now will you bear with me for five minutes will I endeavour to 
apply these results to cometary theory ? I am encouraged to do so bv a remark 
of Bessel's, who said that had any theory preceded his observations on Halley’s 
comet, by fixing his attention either upon its verification or its confutation, it 
would have enabled him to return from his observations with a greater store of 
knowledge than he had actually derived from them.* If time permitted, I should 
like to lead you by an easy gradient up to the view that I wish to submit to 
you; but time does not permit of this, and therefore the speculation must 
suffer from the baldness arising from the absence of such preparation. 

1 Y'ou are doubtless aware of the tremendous difficulties which beset cometary 
theory. The comet examined by Newton in 1680 shot out a tail sixty millions 
of miles in length in two days. The comet of 1843, if I remember aright, shot 
out in a single day a tail which covered 100 degrees of the heavens. This 
enormous reach of cloudy matter is supposed to be generated in the head of the 
comet and driven backwards by some mysterious force of repulsion exerted by 
the sun. Bessel devised a kind of magnetic polarity and repulsion to account 
for it. “It is clear,” says Sir John Herschel, “that*/ we have to deal here 
with matter such as we conceive it, viz. possessing inertia , at all , it must be under 
the dominion of forces incomparably more energetic than gravitation, and quite 

* The remark of Bessel here referred to I found in PoggendorfFs Annalen , vol. xxxviii. 
p. 499. These are his words:—* Ich glaube namlich, dass wir weit brauchbarere Beobaeh- 
tungen Uber die lieschaffenheit der Kometen besitzen wiirden, als wir wirklich besitzen, 
wenn eine Erklarung der Beobachtungen vorh.mden gewesen ware, an welcher sich der 
VViderspruch oder die Bestatigung batten halten kbnnen.’ 


ON COMET ARY THEORY. 443 

of a different nature.” And in another place he states the difficulties of the 
subject in the following remarkable words :— 

‘ “ There is beyond question some profound secret and mystery of nature con¬ 
cerned in the phenomenon of their tails. Perhaps it is not too much to hope 
that future observation, borrowing every aid from rational speculation, grounded 
on the progress of physical science generally (especially those branches of it 
which relate to the sethereal or imponderable elements), may ere long enable 
us to penetrate this mystery, and to declare whether it is really matter, in the 
ordinary acceptation of the term, which is projected from their heads with such 
extravagant velocity, and if not impelled, at least directed in its course by a 
reference to the sun at its point of avoidance. In no respect is this question as 
to the materiality of the tail more forcibly pressed on us for consideration than 
in that of the enormous sweep which it makes round the sun inperilielio , in the 
manner of a straight and rigid rod, in defiance of the law of gravitation, nay, even 
of the received laws of motion, extending (as w^e have seen in the comets of 16S0 
and 1843) from near the sun’s surface to the earth’s orbit, yet whirled round 
unbroken—in the latter case through an angle of 180° in little more than two 
hours It seems utterly incredible that in such a case it is one and the same 
material object which is thus brandished. [I would especially invite the 
reader’s attention to these words in reference to the following theorv.—J. T.~j 
If there could be conceived such a thing as a negative shadow , a momentary 
impression made upon the luminiferous aether behind the comet, this would 
represent in some degree the conception such a phenomenon irresistibly calls 
up.” 

‘ I now ask for permission to lay before you a speculation which seems to do 
away with all these difficulties, and which, whether it represents a physical 
verity or not, ties together the phenomena exhibited by comets in a remark¬ 
ably satisfactory way. 

1 1. The theory is, that a comet is composed of vapour decomposable by the 
solar light, the visible head and tail being an actinic cloud resulting from such 
decomposition; the texture of actinic clouds is demonstrably that of a comet. 

‘2. The tail, according to this theory, is not projected matter, but matter pre¬ 
cipitated on the solar beams traversing the cometary atmosphere. It can be 
proved by experiment that this precipitation may occur either with comparative 
slowness along the beam, or that it may be practically momentary throughout 
the entire length of the beam. The amazing rapidity of the development of 
the tail w r ould be thus accounted for without invoking the incredible motion of 
translation hitherto assumed. 

‘3. As the comet wheels round its perihelion, the tail is not composed through¬ 
out of the same matter, but of new matter precipitated on the solar beams, 
which cross the cometary atmosphere in new directions. The enormous whirl¬ 
ing of the tail is thus accounted for without invoking a motion of translation. 

‘ 4. The tail is always turned from the sun for this reason:—Two anta¬ 
gonistic powers are brought to bear upon the cometary vapour,—the one an 
actinic power, tending to produce precipitation; the other a calomfc power, 
tending to effect vaporization. Where the former prevails, we have the come- 
tary cloud; where the latter prevails, w r e have the transparent cometary 
vapour. As a matter of fact, the sun emits the two agents here invoked. 
There is nothing whatever hypothetical in the assumption of their existence. 
That precipitation should occur behind the head of the comet, or in the space 


444 


ON COMETARY THEORY. 


occupied by the head’s shadow, it is only necessary to assume that the sun’s 
calorific rays are absorbed more copiously by the head and nucleus than the 
actinic rays. This augments the relative superiority of the actinic rays 
behind the head and nucleus, and enables them to bring down the cloud which 
constitutes the comet’s tail. 

‘ 5. The old tail, as it ceases to be screened by the nucleus, is dissipated by 
the solar heat; but its dissipation is not instantaneous. The tail leans towards 
that portion of space last shaded by the comet’s head, a general fact of obser¬ 
vation being thus accounted for. 

G. In the struggle for mastery of the two classes of rays a temporary advan¬ 
tage, owing to variations of density or some other cause, may be gained by the 
actinic rays even in parts of the cometary atmosphere which are unscreened by 
the nucleus. Occasional lateral streamers, and the apparent emission of feeble 
tails towards the sun, would be thus accounted for. 

7. The shrinking of the head in the vicinity of the sun is caused by the 
beating against it of the calorific waves, which dissipate its attenuated fringe 
and cause its apparent contraction. 

‘ Throughout this theory I have dealt exclusively with true causes, and no 
agency has been invoked which does not rest on the sure basis either of 
observation or experiment. It remains with you to say whether in venturing 
to enunciate it I have transgressed the limits of “ rational speculation.” 

1 ^ ^ ^ ave d° ne 80 > surely I could not have come to a place more certain to 
ensure my speedy correction. If the theory be a mere figment of the mind, 
your Adams and your Stokes (both happily here present), to whom I submit 
the speculation with the view of having it instantly annihilated by astronomy 
and physics, if it merit no better fate, will, I doubt not, effectually discharge 
that duty, and thus save both you and me from error before it has had time to 
lay any serious hold on our imagination.’* 

* There may be comets whose vapour is undecomposable by the sun, or which, if decom¬ 
posed, is not precipitated. This view opens out the possibility of invisible comets wandering- 
through space, perhaps sweeping over the earth and affecting its sanitary condition without 
our being otherwise conscious of their passage. As regards tenuity, I entertain a strong 
persuasion that out of a few ounces (the possible wei-ht assigned by Sir John Herschel to 
certain comets) of lodide-of-allyl vapour, an actinic cloud of the magnitude and luminous¬ 
ness of Donati s comet might be manufactured. 


XVII. 


ON THE FORMATION AND PHENOMENA OF CLOUDS.* 

It is well known that when a receiver filled with ordinary undried air is 
exhausted, a cloudiness, due to the precipitation of the aqueous vapour diffused 
m the air, is produced by the first few strokes of the pump. It is, as might be 

expected, possible to produce clouds in this way with the vapours of other 
liquids than water. 

In the course of the experiments on the chemical action of light which have 
been already communicated in abstract to the Royal Society, I had frequent 
occasion to observe the precipitation of such clouds in the experimental tubes 
employed; indeed several days at a time have been devoted solely to the gene¬ 
ration and examination of clouds formed by the sudden dilatation of the air in 
the experimental tubes. 

The clouds were generated in two ways r one mode consisted in opening the 
passage between the filled experimental tube and the air-pump, and then 
simply dilating the air by working the pump. In the other, the experimental 
tube was connected with a vessel of suitable size, the passage between which 
and the experimental tube could be closed by a stopcock. This vessel was first 
exhausted; on turning the cock the air rushed from the experimental tube 
into the vessel, the precipitation of a cloud within the tube being a consequence 
of the transfer. Instead of a special vessel, the cylinders of the air-pump itself 
were usually employed for this purpose. 

It was found possible, by shutting off the residue of air und vapour after each 
act of precipitation, and again exhausting the cylinders of the pump, to obtain 
with some substances, and without refilling the experimental tube, fifteen or 
twenty clouds in succession. * 

The clouds thus precipitated differed from each other in luminous energy, 
some shedding forth a mild white light, others Hashing out with sudden and 
surprising brilliancy. This difference of action is, of course, to be referred to 
the different reflective energies of the particles of the clouds, which were pro¬ 
duced by substances of very different refractive indices. 

Different clouds, moreover, possess very different degrees of stability; some 
melt away rapidly, while others linger for minutes in the experimental tube, 
resting upon its bottom as they dissolve like a heap of snow. The particles of 
other clouds are trailed through the experimental tube as if they were moving 
through a viscous medium. 

Nothing can exceed the splendour of the diffraction-phenomena exhibited bv 
some of these clouds; the colours are best seen by looking along the experi- 


* Proceedings of the Royal Society, No. 110, 1869, 


446 THE FORMATION AND PHENOMENA OF CLOUDS. 


mental tube from a point above it, the face being turned towards the source of 
illumination. The differential m tions introduced by friction against the in¬ 
terior surface of the tube often cause the colours to arrange themselves in 
distinct layers. 

The difference in texture exhibited by different clouds caused me to look a 
little more closely than I had previously done into the mechanism of cloud- 
formation. A certain expansion is necessary to bring down the cloud; the 
moment before precipitation the mass of cooling air and vapour may be 
regarded as divided into a number of polyhedra, the particles along the 
bounding surfaces of which move in opposite directions when precipitation 
actually sets in. Every cloud-particle has consumed a polyhedron of vapour in 
its formation; and it is manifest that the size of the particle must depend, not 
only on the size of the vapour polyhedron, but also on the relation of the den¬ 
sity of the vapour to that of its liquid. If the vapour were light, and the 
liquid heavy, other things being equal, the cloud-particle would be smaller than 
if the vapour were heavy and the liquid light. There would evidently be more 
shrinkage in the one case than in the other : these considerations were found 
valid throughout the experiments; the case of toluol may be taken as repre¬ 
sentative of a great number of others. The specific gravity of this liquid is 
0*85, that of water being unity; the specific gravity of its vapour is 3-20, that 
of aqueous vapour being 0 G. Now, as the size of the cloud-particle is directly 
proportional to the specific gravity of the vapour, and inversely proportional to 
the specific gravity of the liquid, an easy calculation proves that, assuming 
the size of the vapour polyhedra in both cases to be the same, the size of the 
particle of toluol cloud must be more than six times that of the particle of 
aqueous cloud. It is probably impossible to test this question with numerical 
accuracy; but the comparative coarseness of the toluol cloud is strikingly 
manifest to the naked eye. The case is, as I have said, representative. 

In fact, aqueous vapour is without a parallel in these particulars; it is not 
only the lightest of all vapours, in the common acceptation of that term, but 
the lightest of all gases, except hydrogefi and ammonia. To this circumstance 
the soft and tender beauty of the clouds of our atmosphere is mainly to be 
ascribed. 

The sphericity of the cloud-particles may be immediately inferred from their 
deportment under the luminous beam. The light which they shed when 
spherical is continuous but clouds may also be precipitated in solid flakes ; and 
then the incessant sparkling of the cloud shows that its particles are plates , and 
not spheres. Some portions of the same cloud may be composed of spherical 
particles, others of flakes, the difference being at once manifested through the 
calmness of the one portion of the cloud, and the uneasiness of the other. The 
sparkling of such flakes reminded me of the plates of mica in the River Rhone 
at its entrance into the lake of Oeneva, when shone upon by a strong sun. 


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A TREATISE OS ETHNOLOGY. 

15y tine Rev. EDWARD FONTA B^’E, 

PROFESSOR OF THEOLOGY AND NATURAL 8CIENCE * A MEMBER OP TIIE NEW YORK HISTORICAL 
SOCIETY, AND OF THE ACADEMIES OF SCIENCES OF NEW ORLEANS, BALTIMORE, ETC. 

This learned, but simple and intelligible, book is the result of more than thirty years’ careful 
study of anrient and modern history, and the archaeology of all nations. By his independent re¬ 
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natural history, as far as those sciences have been matured by others, he has succeeded, he thinks, 
in so arranging incontrovertible facts as to settle satisfactorily the disputed question of the origin 
and antiquity of mankind. His researches have, brought him to the conclusion of Alexander von 
Ilumboldt, and Jjis brother William, that we are all the descendants of one originally created pair, 
as the Bible teaches. This placeshim in opposition to Sir Ttoderick Murchison, Professor Agassiz, 
Mr. Darwin, and many other eminent naturalists. Mr. Fontaine’s book will commend itself 
especially to students of theology, clergymen of all churches, and the professors of colleges. 
Every argument heretofore advanced against the biblical account of the origin of man is fairly 
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and honest reasoner can read the book carefully without coming to the conclusion that the Mosaic 
cosmogony is correct, and that the account given in Genesis of the origin of mankind is true. 




























































































































































































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